theory Adaption
imports Setup
begin
section {* Adaption to target languages \label{sec:adaption} *}
subsection {* Common adaption cases *}
text {*
The @{theory HOL} @{theory Main} theory already provides a code
generator setup
which should be suitable for most applications. Common extensions
and modifications are available by certain theories of the @{text HOL}
library; beside being useful in applications, they may serve
as a tutorial for customising the code generator setup (see below
\secref{sec:adaption_mechanisms}).
\begin{description}
\item[@{theory "Code_Integer"}] represents @{text HOL} integers by big
integer literals in target languages.
\item[@{theory "Code_Char"}] represents @{text HOL} characters by
character literals in target languages.
\item[@{theory "Code_Char_chr"}] like @{text "Code_Char"},
but also offers treatment of character codes; includes
@{theory "Code_Char_chr"}.
\item[@{theory "Efficient_Nat"}] \label{eff_nat} implements natural numbers by integers,
which in general will result in higher efficiency; pattern
matching with @{term "0\<Colon>nat"} / @{const "Suc"}
is eliminated; includes @{theory "Code_Integer"}.
\item[@{theory "Code_Index"}] provides an additional datatype
@{typ index} which is mapped to target-language built-in integers.
Useful for code setups which involve e.g. indexing of
target-language arrays.
\item[@{theory "Code_Message"}] provides an additional datatype
@{typ message_string} which is isomorphic to strings;
@{typ message_string}s are mapped to target-language strings.
Useful for code setups which involve e.g. printing (error) messages.
\end{description}
\begin{warn}
When importing any of these theories, they should form the last
items in an import list. Since these theories adapt the
code generator setup in a non-conservative fashion,
strange effects may occur otherwise.
\end{warn}
*}
subsection {* Adaption mechanisms \label{sec:adaption_mechanisms} *}
text {*
\begin{warn}
The mechanisms shown here are especially for the curious; the user
rarely needs to do anything on his own beyond the defaults in @{text HOL}.
Adaption is a delicated task which requires a lot of dilligence since
it happend \emph{completely} outside the logic.
\end{warn}
*}
text {*
Consider the following function and its corresponding
SML code:
*}
primrec %quoteme in_interval :: "nat \<times> nat \<Rightarrow> nat \<Rightarrow> bool" where
"in_interval (k, l) n \<longleftrightarrow> k \<le> n \<and> n \<le> l"
(*<*)
code_type %invisible bool
(SML)
code_const %invisible True and False and "op \<and>" and Not
(SML and and and)
(*>*)
text %quoteme {*@{code_stmts in_interval (SML)}*}
text {*
\noindent Though this is correct code, it is a little bit unsatisfactory:
boolean values and operators are materialised as distinguished
entities with have nothing to do with the SML-built-in notion
of \qt{bool}. This results in less readable code;
additionally, eager evaluation may cause programs to
loop or break which would perfectly terminate when
the existing SML @{verbatim "bool"} would be used. To map
the HOL @{typ bool} on SML @{verbatim "bool"}, we may use
\qn{custom serialisations}:
*}
code_type %tt bool
(SML "bool")
code_const %tt True and False and "op \<and>"
(SML "true" and "false" and "_ andalso _")
text {*
\noindent The @{command code_type} command takes a type constructor
as arguments together with a list of custom serialisations.
Each custom serialisation starts with a target language
identifier followed by an expression, which during
code serialisation is inserted whenever the type constructor
would occur. For constants, @{command code_const} implements
the corresponding mechanism. Each ``@{verbatim "_"}'' in
a serialisation expression is treated as a placeholder
for the type constructor's (the constant's) arguments.
*}
text %quoteme {*@{code_stmts in_interval (SML)}*}
text {*
\noindent This still is not perfect: the parentheses
around the \qt{andalso} expression are superfluous.
Though the serializer
by no means attempts to imitate the rich Isabelle syntax
framework, it provides some common idioms, notably
associative infixes with precedences which may be used here:
*}
code_const %tt "op \<and>"
(SML infixl 1 "andalso")
text %quoteme {*@{code_stmts in_interval (SML)}*}
text {*
\noindent Next, we try to map HOL pairs to SML pairs, using the
infix ``@{verbatim "*"}'' type constructor and parentheses:
*}
(*<*)
code_type %invisible *
(SML)
code_const %invisible Pair
(SML)
(*>*)
code_type %tt *
(SML infix 2 "*")
code_const %tt Pair
(SML "!((_),/ (_))")
text {*
\noindent The initial bang ``@{verbatim "!"}'' tells the serializer to never put
parentheses around the whole expression (they are already present),
while the parentheses around argument place holders
tell not to put parentheses around the arguments.
The slash ``@{verbatim "/"}'' (followed by arbitrary white space)
inserts a space which may be used as a break if necessary
during pretty printing.
These examples give a glimpse what mechanisms
custom serialisations provide; however their usage
requires careful thinking in order not to introduce
inconsistencies -- or, in other words:
custom serialisations are completely axiomatic.
A further noteworthy details is that any special
character in a custom serialisation may be quoted
using ``@{verbatim "'"}''; thus, in
``@{verbatim "fn '_ => _"}'' the first
``@{verbatim "_"}'' is a proper underscore while the
second ``@{verbatim "_"}'' is a placeholder.
The HOL theories provide further
examples for custom serialisations.
*}
subsection {* @{text Haskell} serialisation *}
text {*
For convenience, the default
@{text HOL} setup for @{text Haskell} maps the @{class eq} class to
its counterpart in @{text Haskell}, giving custom serialisations
for the class @{class eq} (by command @{command code_class}) and its operation
@{const HOL.eq}
*}
code_class %tt eq
(Haskell "Eq" where "HOL.eq" \<equiv> "(==)")
code_const %tt "op ="
(Haskell infixl 4 "==")
text {*
\noindent A problem now occurs whenever a type which
is an instance of @{class eq} in @{text HOL} is mapped
on a @{text Haskell}-built-in type which is also an instance
of @{text Haskell} @{text Eq}:
*}
typedecl %quoteme bar
instantiation %quoteme bar :: eq
begin
definition %quoteme "eq_class.eq (x\<Colon>bar) y \<longleftrightarrow> x = y"
instance %quoteme by default (simp add: eq_bar_def)
end %quoteme
code_type %tt bar
(Haskell "Integer")
text {*
\noindent The code generator would produce
an additional instance, which of course is rejectedby the @{text Haskell}
compiler.
To suppress this additional instance, use
@{text "code_instance"}:
*}
code_instance %tt bar :: eq
(Haskell -)
end