theory Prelim
imports Base
begin
chapter {* Preliminaries *}
section {* Contexts \label{sec:context} *}
text {*
A logical context represents the background that is required for
formulating statements and composing proofs. It acts as a medium to
produce formal content, depending on earlier material (declarations,
results etc.).
For example, derivations within the Isabelle/Pure logic can be
described as a judgment @{text "\<Gamma> \<turnstile>\<^sub>\<Theta> \<phi>"}, which means that a
proposition @{text "\<phi>"} is derivable from hypotheses @{text "\<Gamma>"}
within the theory @{text "\<Theta>"}. There are logical reasons for
keeping @{text "\<Theta>"} and @{text "\<Gamma>"} separate: theories can be
liberal about supporting type constructors and schematic
polymorphism of constants and axioms, while the inner calculus of
@{text "\<Gamma> \<turnstile> \<phi>"} is strictly limited to Simple Type Theory (with
fixed type variables in the assumptions).
\medskip Contexts and derivations are linked by the following key
principles:
\begin{itemize}
\item Transfer: monotonicity of derivations admits results to be
transferred into a \emph{larger} context, i.e.\ @{text "\<Gamma> \<turnstile>\<^sub>\<Theta>
\<phi>"} implies @{text "\<Gamma>' \<turnstile>\<^sub>\<Theta>\<^sub>' \<phi>"} for contexts @{text "\<Theta>'
\<supseteq> \<Theta>"} and @{text "\<Gamma>' \<supseteq> \<Gamma>"}.
\item Export: discharge of hypotheses admits results to be exported
into a \emph{smaller} context, i.e.\ @{text "\<Gamma>' \<turnstile>\<^sub>\<Theta> \<phi>"}
implies @{text "\<Gamma> \<turnstile>\<^sub>\<Theta> \<Delta> \<Longrightarrow> \<phi>"} where @{text "\<Gamma>' \<supseteq> \<Gamma>"} and
@{text "\<Delta> = \<Gamma>' - \<Gamma>"}. Note that @{text "\<Theta>"} remains unchanged here,
only the @{text "\<Gamma>"} part is affected.
\end{itemize}
\medskip By modeling the main characteristics of the primitive
@{text "\<Theta>"} and @{text "\<Gamma>"} above, and abstracting over any
particular logical content, we arrive at the fundamental notions of
\emph{theory context} and \emph{proof context} in Isabelle/Isar.
These implement a certain policy to manage arbitrary \emph{context
data}. There is a strongly-typed mechanism to declare new kinds of
data at compile time.
The internal bootstrap process of Isabelle/Pure eventually reaches a
stage where certain data slots provide the logical content of @{text
"\<Theta>"} and @{text "\<Gamma>"} sketched above, but this does not stop there!
Various additional data slots support all kinds of mechanisms that
are not necessarily part of the core logic.
For example, there would be data for canonical introduction and
elimination rules for arbitrary operators (depending on the
object-logic and application), which enables users to perform
standard proof steps implicitly (cf.\ the @{text "rule"} method
\cite{isabelle-isar-ref}).
\medskip Thus Isabelle/Isar is able to bring forth more and more
concepts successively. In particular, an object-logic like
Isabelle/HOL continues the Isabelle/Pure setup by adding specific
components for automated reasoning (classical reasoner, tableau
prover, structured induction etc.) and derived specification
mechanisms (inductive predicates, recursive functions etc.). All of
this is ultimately based on the generic data management by theory
and proof contexts introduced here.
*}
subsection {* Theory context \label{sec:context-theory} *}
text {* A \emph{theory} is a data container with explicit name and
unique identifier. Theories are related by a (nominal) sub-theory
relation, which corresponds to the dependency graph of the original
construction; each theory is derived from a certain sub-graph of
ancestor theories. To this end, the system maintains a set of
symbolic ``identification stamps'' within each theory.
In order to avoid the full-scale overhead of explicit sub-theory
identification of arbitrary intermediate stages, a theory is
switched into @{text "draft"} mode under certain circumstances. A
draft theory acts like a linear type, where updates invalidate
earlier versions. An invalidated draft is called \emph{stale}.
The @{text "checkpoint"} operation produces a safe stepping stone
that will survive the next update without becoming stale: both the
old and the new theory remain valid and are related by the
sub-theory relation. Checkpointing essentially recovers purely
functional theory values, at the expense of some extra internal
bookkeeping.
The @{text "copy"} operation produces an auxiliary version that has
the same data content, but is unrelated to the original: updates of
the copy do not affect the original, neither does the sub-theory
relation hold.
The @{text "merge"} operation produces the least upper bound of two
theories, which actually degenerates into absorption of one theory
into the other (according to the nominal sub-theory relation).
The @{text "begin"} operation starts a new theory by importing
several parent theories and entering a special mode of nameless
incremental updates, until the final @{text "end"} operation is
performed.
\medskip The example in \figref{fig:ex-theory} below shows a theory
graph derived from @{text "Pure"}, with theory @{text "Length"}
importing @{text "Nat"} and @{text "List"}. The body of @{text
"Length"} consists of a sequence of updates, working mostly on
drafts internally, while transaction boundaries of Isar top-level
commands (\secref{sec:isar-toplevel}) are guaranteed to be safe
checkpoints.
\begin{figure}[htb]
\begin{center}
\begin{tabular}{rcccl}
& & @{text "Pure"} \\
& & @{text "\<down>"} \\
& & @{text "FOL"} \\
& $\swarrow$ & & $\searrow$ & \\
@{text "Nat"} & & & & @{text "List"} \\
& $\searrow$ & & $\swarrow$ \\
& & @{text "Length"} \\
& & \multicolumn{3}{l}{~~@{keyword "imports"}} \\
& & \multicolumn{3}{l}{~~@{keyword "begin"}} \\
& & $\vdots$~~ \\
& & @{text "\<bullet>"}~~ \\
& & $\vdots$~~ \\
& & @{text "\<bullet>"}~~ \\
& & $\vdots$~~ \\
& & \multicolumn{3}{l}{~~@{command "end"}} \\
\end{tabular}
\caption{A theory definition depending on ancestors}\label{fig:ex-theory}
\end{center}
\end{figure}
\medskip There is a separate notion of \emph{theory reference} for
maintaining a live link to an evolving theory context: updates on
drafts are propagated automatically. Dynamic updating stops when
the next @{text "checkpoint"} is reached.
Derived entities may store a theory reference in order to indicate
the formal context from which they are derived. This implicitly
assumes monotonic reasoning, because the referenced context may
become larger without further notice.
*}
text %mlref {*
\begin{mldecls}
@{index_ML_type theory} \\
@{index_ML Theory.eq_thy: "theory * theory -> bool"} \\
@{index_ML Theory.subthy: "theory * theory -> bool"} \\
@{index_ML Theory.checkpoint: "theory -> theory"} \\
@{index_ML Theory.copy: "theory -> theory"} \\
@{index_ML Theory.merge: "theory * theory -> theory"} \\
@{index_ML Theory.begin_theory: "string -> theory list -> theory"} \\
@{index_ML Theory.parents_of: "theory -> theory list"} \\
@{index_ML Theory.ancestors_of: "theory -> theory list"} \\
\end{mldecls}
\begin{mldecls}
@{index_ML_type theory_ref} \\
@{index_ML Theory.deref: "theory_ref -> theory"} \\
@{index_ML Theory.check_thy: "theory -> theory_ref"} \\
\end{mldecls}
\begin{description}
\item Type @{ML_type theory} represents theory contexts. This is
essentially a linear type, with explicit runtime checking.
Primitive theory operations destroy the original version, which then
becomes ``stale''. This can be prevented by explicit checkpointing,
which the system does at least at the boundary of toplevel command
transactions \secref{sec:isar-toplevel}.
\item @{ML "Theory.eq_thy"}~@{text "(thy\<^sub>1, thy\<^sub>2)"} check strict
identity of two theories.
\item @{ML "Theory.subthy"}~@{text "(thy\<^sub>1, thy\<^sub>2)"} compares theories
according to the intrinsic graph structure of the construction.
This sub-theory relation is a nominal approximation of inclusion
(@{text "\<subseteq>"}) of the corresponding content (according to the
semantics of the ML modules that implement the data).
\item @{ML "Theory.checkpoint"}~@{text "thy"} produces a safe
stepping stone in the linear development of @{text "thy"}. This
changes the old theory, but the next update will result in two
related, valid theories.
\item @{ML "Theory.copy"}~@{text "thy"} produces a variant of @{text
"thy"} with the same data. The copy is not related to the original,
but the original is unchanged.
\item @{ML "Theory.merge"}~@{text "(thy\<^sub>1, thy\<^sub>2)"} absorbs one theory
into the other, without changing @{text "thy\<^sub>1"} or @{text "thy\<^sub>2"}.
This version of ad-hoc theory merge fails for unrelated theories!
\item @{ML "Theory.begin_theory"}~@{text "name parents"} constructs
a new theory based on the given parents. This ML function is
normally not invoked directly.
\item @{ML "Theory.parents_of"}~@{text "thy"} returns the direct
ancestors of @{text thy}.
\item @{ML "Theory.ancestors_of"}~@{text "thy"} returns all
ancestors of @{text thy} (not including @{text thy} itself).
\item Type @{ML_type theory_ref} represents a sliding reference to
an always valid theory; updates on the original are propagated
automatically.
\item @{ML "Theory.deref"}~@{text "thy_ref"} turns a @{ML_type
"theory_ref"} into an @{ML_type "theory"} value. As the referenced
theory evolves monotonically over time, later invocations of @{ML
"Theory.deref"} may refer to a larger context.
\item @{ML "Theory.check_thy"}~@{text "thy"} produces a @{ML_type
"theory_ref"} from a valid @{ML_type "theory"} value.
\end{description}
*}
text %mlantiq {*
\begin{matharray}{rcl}
@{ML_antiquotation_def "theory"} & : & @{text ML_antiquotation} \\
\end{matharray}
\begin{rail}
'theory' nameref?
;
\end{rail}
\begin{description}
\item @{text "@{theory}"} refers to the background theory of the
current context --- as abstract value.
\item @{text "@{theory A}"} refers to an explicitly named ancestor
theory @{text "A"} of the background theory of the current context
--- as abstract value.
\end{description}
*}
subsection {* Proof context \label{sec:context-proof} *}
text {* A proof context is a container for pure data with a
back-reference to the theory from which it is derived. The @{text
"init"} operation creates a proof context from a given theory.
Modifications to draft theories are propagated to the proof context
as usual, but there is also an explicit @{text "transfer"} operation
to force resynchronization with more substantial updates to the
underlying theory.
Entities derived in a proof context need to record logical
requirements explicitly, since there is no separate context
identification or symbolic inclusion as for theories. For example,
hypotheses used in primitive derivations (cf.\ \secref{sec:thms})
are recorded separately within the sequent @{text "\<Gamma> \<turnstile> \<phi>"}, just to
make double sure. Results could still leak into an alien proof
context due to programming errors, but Isabelle/Isar includes some
extra validity checks in critical positions, notably at the end of a
sub-proof.
Proof contexts may be manipulated arbitrarily, although the common
discipline is to follow block structure as a mental model: a given
context is extended consecutively, and results are exported back
into the original context. Note that an Isar proof state models
block-structured reasoning explicitly, using a stack of proof
contexts internally. For various technical reasons, the background
theory of an Isar proof state must not be changed while the proof is
still under construction!
*}
text %mlref {*
\begin{mldecls}
@{index_ML_type Proof.context} \\
@{index_ML ProofContext.init_global: "theory -> Proof.context"} \\
@{index_ML ProofContext.theory_of: "Proof.context -> theory"} \\
@{index_ML ProofContext.transfer: "theory -> Proof.context -> Proof.context"} \\
\end{mldecls}
\begin{description}
\item Type @{ML_type Proof.context} represents proof contexts.
Elements of this type are essentially pure values, with a sliding
reference to the background theory.
\item @{ML ProofContext.init_global}~@{text "thy"} produces a proof context
derived from @{text "thy"}, initializing all data.
\item @{ML ProofContext.theory_of}~@{text "ctxt"} selects the
background theory from @{text "ctxt"}, dereferencing its internal
@{ML_type theory_ref}.
\item @{ML ProofContext.transfer}~@{text "thy ctxt"} promotes the
background theory of @{text "ctxt"} to the super theory @{text
"thy"}.
\end{description}
*}
text %mlantiq {*
\begin{matharray}{rcl}
@{ML_antiquotation_def "context"} & : & @{text ML_antiquotation} \\
\end{matharray}
\begin{description}
\item @{text "@{context}"} refers to \emph{the} context at
compile-time --- as abstract value. Independently of (local) theory
or proof mode, this always produces a meaningful result.
This is probably the most common antiquotation in interactive
experimentation with ML inside Isar.
\end{description}
*}
subsection {* Generic contexts \label{sec:generic-context} *}
text {*
A generic context is the disjoint sum of either a theory or proof
context. Occasionally, this enables uniform treatment of generic
context data, typically extra-logical information. Operations on
generic contexts include the usual injections, partial selections,
and combinators for lifting operations on either component of the
disjoint sum.
Moreover, there are total operations @{text "theory_of"} and @{text
"proof_of"} to convert a generic context into either kind: a theory
can always be selected from the sum, while a proof context might
have to be constructed by an ad-hoc @{text "init"} operation, which
incurs a small runtime overhead.
*}
text %mlref {*
\begin{mldecls}
@{index_ML_type Context.generic} \\
@{index_ML Context.theory_of: "Context.generic -> theory"} \\
@{index_ML Context.proof_of: "Context.generic -> Proof.context"} \\
\end{mldecls}
\begin{description}
\item Type @{ML_type Context.generic} is the direct sum of @{ML_type
"theory"} and @{ML_type "Proof.context"}, with the datatype
constructors @{ML "Context.Theory"} and @{ML "Context.Proof"}.
\item @{ML Context.theory_of}~@{text "context"} always produces a
theory from the generic @{text "context"}, using @{ML
"ProofContext.theory_of"} as required.
\item @{ML Context.proof_of}~@{text "context"} always produces a
proof context from the generic @{text "context"}, using @{ML
"ProofContext.init_global"} as required (note that this re-initializes the
context data with each invocation).
\end{description}
*}
subsection {* Context data \label{sec:context-data} *}
text {* The main purpose of theory and proof contexts is to manage
arbitrary (pure) data. New data types can be declared incrementally
at compile time. There are separate declaration mechanisms for any
of the three kinds of contexts: theory, proof, generic.
\paragraph{Theory data} declarations need to implement the following
SML signature:
\medskip
\begin{tabular}{ll}
@{text "\<type> T"} & representing type \\
@{text "\<val> empty: T"} & empty default value \\
@{text "\<val> extend: T \<rightarrow> T"} & re-initialize on import \\
@{text "\<val> merge: T \<times> T \<rightarrow> T"} & join on import \\
\end{tabular}
\medskip
The @{text "empty"} value acts as initial default for \emph{any}
theory that does not declare actual data content; @{text "extend"}
is acts like a unitary version of @{text "merge"}.
Implementing @{text "merge"} can be tricky. The general idea is
that @{text "merge (data\<^sub>1, data\<^sub>2)"} inserts those parts of @{text
"data\<^sub>2"} into @{text "data\<^sub>1"} that are not yet present, while
keeping the general order of things. The @{ML Library.merge}
function on plain lists may serve as canonical template.
Particularly note that shared parts of the data must not be
duplicated by naive concatenation, or a theory graph that is like a
chain of diamonds would cause an exponential blowup!
\paragraph{Proof context data} declarations need to implement the
following SML signature:
\medskip
\begin{tabular}{ll}
@{text "\<type> T"} & representing type \\
@{text "\<val> init: theory \<rightarrow> T"} & produce initial value \\
\end{tabular}
\medskip
The @{text "init"} operation is supposed to produce a pure value
from the given background theory and should be somehow
``immediate''. Whenever a proof context is initialized, which
happens frequently, the the system invokes the @{text "init"}
operation of \emph{all} theory data slots ever declared. This also
means that one needs to be economic about the total number of proof
data declarations in the system, i.e.\ each ML module should declare
at most one, sometimes two data slots for its internal use.
Repeated data declarations to simulate a record type should be
avoided!
\paragraph{Generic data} provides a hybrid interface for both theory
and proof data. The @{text "init"} operation for proof contexts is
predefined to select the current data value from the background
theory.
\bigskip Any of the above data declarations over type @{text "T"}
result in an ML structure with the following signature:
\medskip
\begin{tabular}{ll}
@{text "get: context \<rightarrow> T"} \\
@{text "put: T \<rightarrow> context \<rightarrow> context"} \\
@{text "map: (T \<rightarrow> T) \<rightarrow> context \<rightarrow> context"} \\
\end{tabular}
\medskip
These other operations provide exclusive access for the particular
kind of context (theory, proof, or generic context). This interface
observes the ML discipline for types and scopes: there is no other
way to access the corresponding data slot of a context. By keeping
these operations private, an Isabelle/ML module may maintain
abstract values authentically. *}
text %mlref {*
\begin{mldecls}
@{index_ML_functor Theory_Data} \\
@{index_ML_functor Proof_Data} \\
@{index_ML_functor Generic_Data} \\
\end{mldecls}
\begin{description}
\item @{ML_functor Theory_Data}@{text "(spec)"} declares data for
type @{ML_type theory} according to the specification provided as
argument structure. The resulting structure provides data init and
access operations as described above.
\item @{ML_functor Proof_Data}@{text "(spec)"} is analogous to
@{ML_functor Theory_Data} for type @{ML_type Proof.context}.
\item @{ML_functor Generic_Data}@{text "(spec)"} is analogous to
@{ML_functor Theory_Data} for type @{ML_type Context.generic}.
\end{description}
*}
text %mlex {*
The following artificial example demonstrates theory
data: we maintain a set of terms that are supposed to be wellformed
wrt.\ the enclosing theory. The public interface is as follows:
*}
ML {*
signature WELLFORMED_TERMS =
sig
val get: theory -> term list
val add: term -> theory -> theory
end;
*}
text {* The implementation uses private theory data internally, and
only exposes an operation that involves explicit argument checking
wrt.\ the given theory. *}
ML {*
structure Wellformed_Terms: WELLFORMED_TERMS =
struct
structure Terms = Theory_Data
(
type T = term Ord_List.T;
val empty = [];
val extend = I;
fun merge (ts1, ts2) =
Ord_List.union Term_Ord.fast_term_ord ts1 ts2;
);
val get = Terms.get;
fun add raw_t thy =
let
val t = Sign.cert_term thy raw_t;
in
Terms.map (Ord_List.insert Term_Ord.fast_term_ord t) thy
end;
end;
*}
text {* Type @{ML_type "term Ord_List.T"} is used for reasonably
efficient representation of a set of terms: all operations are
linear in the number of stored elements. Here we assume that users
of this module do not care about the declaration order, since that
data structure forces its own arrangement of elements.
Observe how the @{ML_text merge} operation joins the data slots of
the two constituents: @{ML Ord_List.union} prevents duplication of
common data from different branches, thus avoiding the danger of
exponential blowup. Plain list append etc.\ must never be used for
theory data merges!
\medskip Our intended invariant is achieved as follows:
\begin{enumerate}
\item @{ML Wellformed_Terms.add} only admits terms that have passed
the @{ML Sign.cert_term} check of the given theory at that point.
\item Wellformedness in the sense of @{ML Sign.cert_term} is
monotonic wrt.\ the sub-theory relation. So our data can move
upwards in the hierarchy (via extension or merges), and maintain
wellformedness without further checks.
\end{enumerate}
Note that all basic operations of the inference kernel (which
includes @{ML Sign.cert_term}) observe this monotonicity principle,
but other user-space tools don't. For example, fully-featured
type-inference via @{ML Syntax.check_term} (cf.\
\secref{sec:term-check}) is not necessarily monotonic wrt.\ the
background theory, since constraints of term constants can be
modified by later declarations, for example.
In most cases, user-space context data does not have to take such
invariants too seriously. The situation is different in the
implementation of the inference kernel itself, which uses the very
same data mechanisms for types, constants, axioms etc.
*}
subsection {* Configuration options \label{sec:config-options} *}
text {* A \emph{configuration option} is a named optional value of
some basic type (Boolean, integer, string) that is stored in the
context. It is a simple application of general context data
(\secref{sec:context-data}) that is sufficiently common to justify
customized setup, which includes some concrete declarations for
end-users using existing notation for attributes (cf.\
\secref{sec:attributes}).
For example, the predefined configuration option @{attribute
show_types} controls output of explicit type constraints for
variables in printed terms (cf.\ \secref{sec:read-print}). Its
value can be modified within Isar text like this:
*}
declare [[show_types = false]]
-- {* declaration within (local) theory context *}
notepad
begin
note [[show_types = true]]
-- {* declaration within proof (forward mode) *}
term x
have "x = x"
using [[show_types = false]]
-- {* declaration within proof (backward mode) *}
..
end
text {* Configuration options that are not set explicitly hold a
default value that can depend on the application context. This
allows to retrieve the value from another slot within the context,
or fall back on a global preference mechanism, for example.
The operations to declare configuration options and get/map their
values are modeled as direct replacements for historic global
references, only that the context is made explicit. This allows
easy configuration of tools, without relying on the execution order
as required for old-style mutable references. *}
text %mlref {*
\begin{mldecls}
@{index_ML Config.get: "Proof.context -> 'a Config.T -> 'a"} \\
@{index_ML Config.map: "'a Config.T -> ('a -> 'a) -> Proof.context -> Proof.context"} \\
@{index_ML Attrib.config_bool: "string -> (Context.generic -> bool) ->
bool Config.T * (theory -> theory)"} \\
@{index_ML Attrib.config_int: "string -> (Context.generic -> int) ->
int Config.T * (theory -> theory)"} \\
@{index_ML Attrib.config_real: "string -> (Context.generic -> real) ->
real Config.T * (theory -> theory)"} \\
@{index_ML Attrib.config_string: "string -> (Context.generic -> string) ->
string Config.T * (theory -> theory)"} \\
\end{mldecls}
\begin{description}
\item @{ML Config.get}~@{text "ctxt config"} gets the value of
@{text "config"} in the given context.
\item @{ML Config.map}~@{text "config f ctxt"} updates the context
by updating the value of @{text "config"}.
\item @{text "(config, setup) ="}~@{ML Attrib.config_bool}~@{text
"name default"} creates a named configuration option of type
@{ML_type bool}, with the given @{text "default"} depending on the
application context. The resulting @{text "config"} can be used to
get/map its value in a given context. The @{text "setup"} function
needs to be applied to the theory initially, in order to make
concrete declaration syntax available to the user.
\item @{ML Attrib.config_int}, @{ML Attrib.config_real}, and @{ML
Attrib.config_string} work like @{ML Attrib.config_bool}, but for
types @{ML_type int} and @{ML_type string}, respectively.
\end{description}
*}
text %mlex {* The following example shows how to declare and use a
Boolean configuration option called @{text "my_flag"} with constant
default value @{ML false}. *}
ML {*
val (my_flag, my_flag_setup) =
Attrib.config_bool "my_flag" (K false)
*}
setup my_flag_setup
text {* Now the user can refer to @{attribute my_flag} in
declarations, while ML tools can retrieve the current value from the
context via @{ML Config.get}. *}
ML_val {* @{assert} (Config.get @{context} my_flag = false) *}
declare [[my_flag = true]]
ML_val {* @{assert} (Config.get @{context} my_flag = true) *}
notepad
begin
{
note [[my_flag = false]]
ML_val {* @{assert} (Config.get @{context} my_flag = false) *}
}
ML_val {* @{assert} (Config.get @{context} my_flag = true) *}
end
text {* Here is another example involving ML type @{ML_type real}
(floating-point numbers). *}
ML {*
val (airspeed_velocity, airspeed_velocity_setup) =
Attrib.config_real "airspeed_velocity" (K 0.0)
*}
setup airspeed_velocity_setup
declare [[airspeed_velocity = 10]]
declare [[airspeed_velocity = 9.9]]
section {* Names \label{sec:names} *}
text {* In principle, a name is just a string, but there are various
conventions for representing additional structure. For example,
``@{text "Foo.bar.baz"}'' is considered as a long name consisting of
qualifier @{text "Foo.bar"} and base name @{text "baz"}. The
individual constituents of a name may have further substructure,
e.g.\ the string ``\verb,\,\verb,<alpha>,'' encodes as a single
symbol.
\medskip Subsequently, we shall introduce specific categories of
names. Roughly speaking these correspond to logical entities as
follows:
\begin{itemize}
\item Basic names (\secref{sec:basic-name}): free and bound
variables.
\item Indexed names (\secref{sec:indexname}): schematic variables.
\item Long names (\secref{sec:long-name}): constants of any kind
(type constructors, term constants, other concepts defined in user
space). Such entities are typically managed via name spaces
(\secref{sec:name-space}).
\end{itemize}
*}
subsection {* Strings of symbols \label{sec:symbols} *}
text {* A \emph{symbol} constitutes the smallest textual unit in
Isabelle --- raw ML characters are normally not encountered at all!
Isabelle strings consist of a sequence of symbols, represented as a
packed string or an exploded list of strings. Each symbol is in
itself a small string, which has either one of the following forms:
\begin{enumerate}
\item a single ASCII character ``@{text "c"}'', for example
``\verb,a,'',
\item a codepoint according to UTF8 (non-ASCII byte sequence),
\item a regular symbol ``\verb,\,\verb,<,@{text "ident"}\verb,>,'',
for example ``\verb,\,\verb,<alpha>,'',
\item a control symbol ``\verb,\,\verb,<^,@{text "ident"}\verb,>,'',
for example ``\verb,\,\verb,<^bold>,'',
\item a raw symbol ``\verb,\,\verb,<^raw:,@{text text}\verb,>,''
where @{text text} consists of printable characters excluding
``\verb,.,'' and ``\verb,>,'', for example
``\verb,\,\verb,<^raw:$\sum_{i = 1}^n$>,'',
\item a numbered raw control symbol ``\verb,\,\verb,<^raw,@{text
n}\verb,>, where @{text n} consists of digits, for example
``\verb,\,\verb,<^raw42>,''.
\end{enumerate}
The @{text "ident"} syntax for symbol names is @{text "letter
(letter | digit)\<^sup>*"}, where @{text "letter = A..Za..z"} and @{text
"digit = 0..9"}. There are infinitely many regular symbols and
control symbols, but a fixed collection of standard symbols is
treated specifically. For example, ``\verb,\,\verb,<alpha>,'' is
classified as a letter, which means it may occur within regular
Isabelle identifiers.
The character set underlying Isabelle symbols is 7-bit ASCII, but
8-bit character sequences are passed-through unchanged. Unicode/UCS
data in UTF-8 encoding is processed in a non-strict fashion, such
that well-formed code sequences are recognized
accordingly.\footnote{Note that ISO-Latin-1 differs from UTF-8 only
in some special punctuation characters that even have replacements
within the standard collection of Isabelle symbols. Text consisting
of ASCII plus accented letters can be processed in either encoding.}
Unicode provides its own collection of mathematical symbols, but
within the core Isabelle/ML world there is no link to the standard
collection of Isabelle regular symbols.
\medskip Output of Isabelle symbols depends on the print mode
(\secref{print-mode}). For example, the standard {\LaTeX} setup of
the Isabelle document preparation system would present
``\verb,\,\verb,<alpha>,'' as @{text "\<alpha>"}, and
``\verb,\,\verb,<^bold>,\verb,\,\verb,<alpha>,'' as @{text
"\<^bold>\<alpha>"}. On-screen rendering usually works by mapping a finite
subset of Isabelle symbols to suitable Unicode characters.
*}
text %mlref {*
\begin{mldecls}
@{index_ML_type "Symbol.symbol": string} \\
@{index_ML Symbol.explode: "string -> Symbol.symbol list"} \\
@{index_ML Symbol.is_letter: "Symbol.symbol -> bool"} \\
@{index_ML Symbol.is_digit: "Symbol.symbol -> bool"} \\
@{index_ML Symbol.is_quasi: "Symbol.symbol -> bool"} \\
@{index_ML Symbol.is_blank: "Symbol.symbol -> bool"} \\
\end{mldecls}
\begin{mldecls}
@{index_ML_type "Symbol.sym"} \\
@{index_ML Symbol.decode: "Symbol.symbol -> Symbol.sym"} \\
\end{mldecls}
\begin{description}
\item Type @{ML_type "Symbol.symbol"} represents individual Isabelle
symbols.
\item @{ML "Symbol.explode"}~@{text "str"} produces a symbol list
from the packed form. This function supersedes @{ML
"String.explode"} for virtually all purposes of manipulating text in
Isabelle!\footnote{The runtime overhead for exploded strings is
mainly that of the list structure: individual symbols that happen to
be a singleton string do not require extra memory in Poly/ML.}
\item @{ML "Symbol.is_letter"}, @{ML "Symbol.is_digit"}, @{ML
"Symbol.is_quasi"}, @{ML "Symbol.is_blank"} classify standard
symbols according to fixed syntactic conventions of Isabelle, cf.\
\cite{isabelle-isar-ref}.
\item Type @{ML_type "Symbol.sym"} is a concrete datatype that
represents the different kinds of symbols explicitly, with
constructors @{ML "Symbol.Char"}, @{ML "Symbol.Sym"}, @{ML
"Symbol.UTF8"}, @{ML "Symbol.Ctrl"}, @{ML "Symbol.Raw"}.
\item @{ML "Symbol.decode"} converts the string representation of a
symbol into the datatype version.
\end{description}
\paragraph{Historical note.} In the original SML90 standard the
primitive ML type @{ML_type char} did not exists, and the @{ML_text
"explode: string -> string list"} operation would produce a list of
singleton strings as does @{ML "raw_explode: string -> string list"}
in Isabelle/ML today. When SML97 came out, Isabelle did not adopt
its slightly anachronistic 8-bit characters, but the idea of
exploding a string into a list of small strings was extended to
``symbols'' as explained above. Thus Isabelle sources can refer to
an infinite store of user-defined symbols, without having to worry
about the multitude of Unicode encodings. *}
subsection {* Basic names \label{sec:basic-name} *}
text {*
A \emph{basic name} essentially consists of a single Isabelle
identifier. There are conventions to mark separate classes of basic
names, by attaching a suffix of underscores: one underscore means
\emph{internal name}, two underscores means \emph{Skolem name},
three underscores means \emph{internal Skolem name}.
For example, the basic name @{text "foo"} has the internal version
@{text "foo_"}, with Skolem versions @{text "foo__"} and @{text
"foo___"}, respectively.
These special versions provide copies of the basic name space, apart
from anything that normally appears in the user text. For example,
system generated variables in Isar proof contexts are usually marked
as internal, which prevents mysterious names like @{text "xaa"} to
appear in human-readable text.
\medskip Manipulating binding scopes often requires on-the-fly
renamings. A \emph{name context} contains a collection of already
used names. The @{text "declare"} operation adds names to the
context.
The @{text "invents"} operation derives a number of fresh names from
a given starting point. For example, the first three names derived
from @{text "a"} are @{text "a"}, @{text "b"}, @{text "c"}.
The @{text "variants"} operation produces fresh names by
incrementing tentative names as base-26 numbers (with digits @{text
"a..z"}) until all clashes are resolved. For example, name @{text
"foo"} results in variants @{text "fooa"}, @{text "foob"}, @{text
"fooc"}, \dots, @{text "fooaa"}, @{text "fooab"} etc.; each renaming
step picks the next unused variant from this sequence.
*}
text %mlref {*
\begin{mldecls}
@{index_ML Name.internal: "string -> string"} \\
@{index_ML Name.skolem: "string -> string"} \\
\end{mldecls}
\begin{mldecls}
@{index_ML_type Name.context} \\
@{index_ML Name.context: Name.context} \\
@{index_ML Name.declare: "string -> Name.context -> Name.context"} \\
@{index_ML Name.invents: "Name.context -> string -> int -> string list"} \\
@{index_ML Name.variants: "string list -> Name.context -> string list * Name.context"} \\
\end{mldecls}
\begin{mldecls}
@{index_ML Variable.names_of: "Proof.context -> Name.context"} \\
\end{mldecls}
\begin{description}
\item @{ML Name.internal}~@{text "name"} produces an internal name
by adding one underscore.
\item @{ML Name.skolem}~@{text "name"} produces a Skolem name by
adding two underscores.
\item Type @{ML_type Name.context} represents the context of already
used names; the initial value is @{ML "Name.context"}.
\item @{ML Name.declare}~@{text "name"} enters a used name into the
context.
\item @{ML Name.invents}~@{text "context name n"} produces @{text
"n"} fresh names derived from @{text "name"}.
\item @{ML Name.variants}~@{text "names context"} produces fresh
variants of @{text "names"}; the result is entered into the context.
\item @{ML Variable.names_of}~@{text "ctxt"} retrieves the context
of declared type and term variable names. Projecting a proof
context down to a primitive name context is occasionally useful when
invoking lower-level operations. Regular management of ``fresh
variables'' is done by suitable operations of structure @{ML_struct
Variable}, which is also able to provide an official status of
``locally fixed variable'' within the logical environment (cf.\
\secref{sec:variables}).
\end{description}
*}
text %mlex {* The following simple examples demonstrate how to produce
fresh names from the initial @{ML Name.context}. *}
ML {*
val list1 = Name.invents Name.context "a" 5;
@{assert} (list1 = ["a", "b", "c", "d", "e"]);
val list2 =
#1 (Name.variants ["x", "x", "a", "a", "'a", "'a"] Name.context);
@{assert} (list2 = ["x", "xa", "a", "aa", "'a", "'aa"]);
*}
text {* \medskip The same works relatively to the formal context as
follows. *}
locale ex = fixes a b c :: 'a
begin
ML {*
val names = Variable.names_of @{context};
val list1 = Name.invents names "a" 5;
@{assert} (list1 = ["d", "e", "f", "g", "h"]);
val list2 =
#1 (Name.variants ["x", "x", "a", "a", "'a", "'a"] names);
@{assert} (list2 = ["x", "xa", "aa", "ab", "'aa", "'ab"]);
*}
end
subsection {* Indexed names \label{sec:indexname} *}
text {*
An \emph{indexed name} (or @{text "indexname"}) is a pair of a basic
name and a natural number. This representation allows efficient
renaming by incrementing the second component only. The canonical
way to rename two collections of indexnames apart from each other is
this: determine the maximum index @{text "maxidx"} of the first
collection, then increment all indexes of the second collection by
@{text "maxidx + 1"}; the maximum index of an empty collection is
@{text "-1"}.
Occasionally, basic names are injected into the same pair type of
indexed names: then @{text "(x, -1)"} is used to encode the basic
name @{text "x"}.
\medskip Isabelle syntax observes the following rules for
representing an indexname @{text "(x, i)"} as a packed string:
\begin{itemize}
\item @{text "?x"} if @{text "x"} does not end with a digit and @{text "i = 0"},
\item @{text "?xi"} if @{text "x"} does not end with a digit,
\item @{text "?x.i"} otherwise.
\end{itemize}
Indexnames may acquire large index numbers after several maxidx
shifts have been applied. Results are usually normalized towards
@{text "0"} at certain checkpoints, notably at the end of a proof.
This works by producing variants of the corresponding basic name
components. For example, the collection @{text "?x1, ?x7, ?x42"}
becomes @{text "?x, ?xa, ?xb"}.
*}
text %mlref {*
\begin{mldecls}
@{index_ML_type indexname: "string * int"} \\
\end{mldecls}
\begin{description}
\item Type @{ML_type indexname} represents indexed names. This is
an abbreviation for @{ML_type "string * int"}. The second component
is usually non-negative, except for situations where @{text "(x,
-1)"} is used to inject basic names into this type. Other negative
indexes should not be used.
\end{description}
*}
subsection {* Long names \label{sec:long-name} *}
text {* A \emph{long name} consists of a sequence of non-empty name
components. The packed representation uses a dot as separator, as
in ``@{text "A.b.c"}''. The last component is called \emph{base
name}, the remaining prefix is called \emph{qualifier} (which may be
empty). The qualifier can be understood as the access path to the
named entity while passing through some nested block-structure,
although our free-form long names do not really enforce any strict
discipline.
For example, an item named ``@{text "A.b.c"}'' may be understood as
a local entity @{text "c"}, within a local structure @{text "b"},
within a global structure @{text "A"}. In practice, long names
usually represent 1--3 levels of qualification. User ML code should
not make any assumptions about the particular structure of long
names!
The empty name is commonly used as an indication of unnamed
entities, or entities that are not entered into the corresponding
name space, whenever this makes any sense. The basic operations on
long names map empty names again to empty names.
*}
text %mlref {*
\begin{mldecls}
@{index_ML Long_Name.base_name: "string -> string"} \\
@{index_ML Long_Name.qualifier: "string -> string"} \\
@{index_ML Long_Name.append: "string -> string -> string"} \\
@{index_ML Long_Name.implode: "string list -> string"} \\
@{index_ML Long_Name.explode: "string -> string list"} \\
\end{mldecls}
\begin{description}
\item @{ML Long_Name.base_name}~@{text "name"} returns the base name
of a long name.
\item @{ML Long_Name.qualifier}~@{text "name"} returns the qualifier
of a long name.
\item @{ML Long_Name.append}~@{text "name\<^isub>1 name\<^isub>2"} appends two long
names.
\item @{ML Long_Name.implode}~@{text "names"} and @{ML
Long_Name.explode}~@{text "name"} convert between the packed string
representation and the explicit list form of long names.
\end{description}
*}
subsection {* Name spaces \label{sec:name-space} *}
text {* A @{text "name space"} manages a collection of long names,
together with a mapping between partially qualified external names
and fully qualified internal names (in both directions). Note that
the corresponding @{text "intern"} and @{text "extern"} operations
are mostly used for parsing and printing only! The @{text
"declare"} operation augments a name space according to the accesses
determined by a given binding, and a naming policy from the context.
\medskip A @{text "binding"} specifies details about the prospective
long name of a newly introduced formal entity. It consists of a
base name, prefixes for qualification (separate ones for system
infrastructure and user-space mechanisms), a slot for the original
source position, and some additional flags.
\medskip A @{text "naming"} provides some additional details for
producing a long name from a binding. Normally, the naming is
implicit in the theory or proof context. The @{text "full"}
operation (and its variants for different context types) produces a
fully qualified internal name to be entered into a name space. The
main equation of this ``chemical reaction'' when binding new
entities in a context is as follows:
\medskip
\begin{tabular}{l}
@{text "binding + naming \<longrightarrow> long name + name space accesses"}
\end{tabular}
\bigskip As a general principle, there is a separate name space for
each kind of formal entity, e.g.\ fact, logical constant, type
constructor, type class. It is usually clear from the occurrence in
concrete syntax (or from the scope) which kind of entity a name
refers to. For example, the very same name @{text "c"} may be used
uniformly for a constant, type constructor, and type class.
There are common schemes to name derived entities systematically
according to the name of the main logical entity involved, e.g.\
fact @{text "c.intro"} for a canonical introduction rule related to
constant @{text "c"}. This technique of mapping names from one
space into another requires some care in order to avoid conflicts.
In particular, theorem names derived from a type constructor or type
class should get an additional suffix in addition to the usual
qualification. This leads to the following conventions for derived
names:
\medskip
\begin{tabular}{ll}
logical entity & fact name \\\hline
constant @{text "c"} & @{text "c.intro"} \\
type @{text "c"} & @{text "c_type.intro"} \\
class @{text "c"} & @{text "c_class.intro"} \\
\end{tabular}
*}
text %mlref {*
\begin{mldecls}
@{index_ML_type binding} \\
@{index_ML Binding.empty: binding} \\
@{index_ML Binding.name: "string -> binding"} \\
@{index_ML Binding.qualify: "bool -> string -> binding -> binding"} \\
@{index_ML Binding.prefix: "bool -> string -> binding -> binding"} \\
@{index_ML Binding.conceal: "binding -> binding"} \\
@{index_ML Binding.str_of: "binding -> string"} \\
\end{mldecls}
\begin{mldecls}
@{index_ML_type Name_Space.naming} \\
@{index_ML Name_Space.default_naming: Name_Space.naming} \\
@{index_ML Name_Space.add_path: "string -> Name_Space.naming -> Name_Space.naming"} \\
@{index_ML Name_Space.full_name: "Name_Space.naming -> binding -> string"} \\
\end{mldecls}
\begin{mldecls}
@{index_ML_type Name_Space.T} \\
@{index_ML Name_Space.empty: "string -> Name_Space.T"} \\
@{index_ML Name_Space.merge: "Name_Space.T * Name_Space.T -> Name_Space.T"} \\
@{index_ML Name_Space.declare: "bool -> Name_Space.naming -> binding -> Name_Space.T ->
string * Name_Space.T"} \\
@{index_ML Name_Space.intern: "Name_Space.T -> string -> string"} \\
@{index_ML Name_Space.extern: "Name_Space.T -> string -> string"} \\
@{index_ML Name_Space.is_concealed: "Name_Space.T -> string -> bool"}
\end{mldecls}
\begin{description}
\item Type @{ML_type binding} represents the abstract concept of
name bindings.
\item @{ML Binding.empty} is the empty binding.
\item @{ML Binding.name}~@{text "name"} produces a binding with base
name @{text "name"}. Note that this lacks proper source position
information; see also the ML antiquotation @{ML_antiquotation
binding}.
\item @{ML Binding.qualify}~@{text "mandatory name binding"}
prefixes qualifier @{text "name"} to @{text "binding"}. The @{text
"mandatory"} flag tells if this name component always needs to be
given in name space accesses --- this is mostly @{text "false"} in
practice. Note that this part of qualification is typically used in
derived specification mechanisms.
\item @{ML Binding.prefix} is similar to @{ML Binding.qualify}, but
affects the system prefix. This part of extra qualification is
typically used in the infrastructure for modular specifications,
notably ``local theory targets'' (see also \chref{ch:local-theory}).
\item @{ML Binding.conceal}~@{text "binding"} indicates that the
binding shall refer to an entity that serves foundational purposes
only. This flag helps to mark implementation details of
specification mechanism etc. Other tools should not depend on the
particulars of concealed entities (cf.\ @{ML
Name_Space.is_concealed}).
\item @{ML Binding.str_of}~@{text "binding"} produces a string
representation for human-readable output, together with some formal
markup that might get used in GUI front-ends, for example.
\item Type @{ML_type Name_Space.naming} represents the abstract
concept of a naming policy.
\item @{ML Name_Space.default_naming} is the default naming policy.
In a theory context, this is usually augmented by a path prefix
consisting of the theory name.
\item @{ML Name_Space.add_path}~@{text "path naming"} augments the
naming policy by extending its path component.
\item @{ML Name_Space.full_name}~@{text "naming binding"} turns a
name binding (usually a basic name) into the fully qualified
internal name, according to the given naming policy.
\item Type @{ML_type Name_Space.T} represents name spaces.
\item @{ML Name_Space.empty}~@{text "kind"} and @{ML Name_Space.merge}~@{text
"(space\<^isub>1, space\<^isub>2)"} are the canonical operations for
maintaining name spaces according to theory data management
(\secref{sec:context-data}); @{text "kind"} is a formal comment
to characterize the purpose of a name space.
\item @{ML Name_Space.declare}~@{text "strict naming bindings
space"} enters a name binding as fully qualified internal name into
the name space, with external accesses determined by the naming
policy.
\item @{ML Name_Space.intern}~@{text "space name"} internalizes a
(partially qualified) external name.
This operation is mostly for parsing! Note that fully qualified
names stemming from declarations are produced via @{ML
"Name_Space.full_name"} and @{ML "Name_Space.declare"}
(or their derivatives for @{ML_type theory} and
@{ML_type Proof.context}).
\item @{ML Name_Space.extern}~@{text "space name"} externalizes a
(fully qualified) internal name.
This operation is mostly for printing! User code should not rely on
the precise result too much.
\item @{ML Name_Space.is_concealed}~@{text "space name"} indicates
whether @{text "name"} refers to a strictly private entity that
other tools are supposed to ignore!
\end{description}
*}
text %mlantiq {*
\begin{matharray}{rcl}
@{ML_antiquotation_def "binding"} & : & @{text ML_antiquotation} \\
\end{matharray}
\begin{rail}
'binding' name
;
\end{rail}
\begin{description}
\item @{text "@{binding name}"} produces a binding with base name
@{text "name"} and the source position taken from the concrete
syntax of this antiquotation. In many situations this is more
appropriate than the more basic @{ML Binding.name} function.
\end{description}
*}
text %mlex {* The following example yields the source position of some
concrete binding inlined into the text:
*}
ML {* Binding.pos_of @{binding here} *}
text {* \medskip That position can be also printed in a message as
follows: *}
ML_command {*
writeln
("Look here" ^ Position.str_of (Binding.pos_of @{binding here}))
*}
text {* This illustrates a key virtue of formalized bindings as
opposed to raw specifications of base names: the system can use this
additional information for feedback given to the user (error
messages etc.). *}
end