doc-src/Codegen/Thy/Evaluation.thy
 author haftmann Sat, 27 Nov 2010 18:51:04 +0100 changeset 40751 6975c4d83ffd parent 40350 1ef7ee8dd165 child 41184 5c6f44d22f51 permissions -rw-r--r--

theory Evaluation
imports Setup
begin

section {* Evaluation \label{sec:evaluation} *}

text {*
Recalling \secref{sec:principle}, code generation turns a system of
equations into a program with the \emph{same} equational semantics.
As a consequence, this program can be used as a \emph{rewrite
engine} for terms: rewriting a term @{term "t"} using a program to a
term @{term "t'"} yields the theorems @{prop "t \<equiv> t'"}.  This
application of code generation in the following is referred to as
\emph{evaluation}.
*}

subsection {* Evaluation techniques *}

text {*
The existing infrastructure provides a rich palette of evaluation
techniques, each comprising different aspects:

\begin{description}

\item[Expressiveness.]  Depending on how good symbolic computation
is supported, the class of terms which can be evaluated may be
bigger or smaller.

\item[Efficiency.]  The more machine-near the technique, the
faster it is.

\item[Trustability.]  Techniques which a huge (and also probably
more configurable infrastructure) are more fragile and less
trustable.

\end{description}
*}

subsubsection {* The simplifier (@{text simp}) *}

text {*
The simplest way for evaluation is just using the simplifier with
the original code equations of the underlying program.  This gives
fully symbolic evaluation and highest trustablity, with the usual
performance of the simplifier.  Note that for operations on abstract
datatypes (cf.~\secref{sec:invariant}), the original theorems as
given by the users are used, not the modified ones.
*}

subsubsection {* Normalization by evaluation (@{text nbe}) *}

text {*
Normalization by evaluation \cite{Aehlig-Haftmann-Nipkow:2008:nbe}
provides a comparably fast partially symbolic evaluation which
permits also normalization of functions and uninterpreted symbols;
the stack of code to be trusted is considerable.
*}

subsubsection {* Evaluation in ML (@{text code}) *}

text {*
Highest performance can be achieved by evaluation in ML, at the cost
of being restricted to ground results and a layered stack of code to
be trusted, including code generator configurations by the user.

Evaluation is carried out in a target language \emph{Eval} which
inherits from \emph{SML} but for convenience uses parts of the
Isabelle runtime environment.  The soundness of computation carried
out there depends crucially on the correctness of the code
generator setup; this is one of the reasons why you should not use
*}

subsection {* Aspects of evaluation *}

text {*
Each of the techniques can be combined with different aspects.  The
most important distinction is between dynamic and static evaluation.
Dynamic evaluation takes the code generator configuration \qt{as it
is} at the point where evaluation is issued.  Best example is the
@{command_def value} command which allows ad-hoc evaluation of
terms:
*}

value %quote "42 / (12 :: rat)"

text {*
\noindent By default @{command value} tries all available evaluation
techniques and prints the result of the first succeeding one.  A particular
technique may be specified in square brackets, e.g.
*}

value %quote [nbe] "42 / (12 :: rat)"

text {*
Static evaluation freezes the code generator configuration at a
certain point and uses this context whenever evaluation is issued
later on.  This is particularly appropriate for proof procedures
which use evaluation, since then the behaviour of evaluation is not
changed or even compromised later on by actions of the user.

As a technical complication, terms after evaluation in ML must be
turned into Isabelle's internal term representation again.  Since
this is also configurable, it is never fully trusted.  For this
reason, evaluation in ML comes with further aspects:

\begin{description}

\item[Plain evaluation.]  A term is normalized using the provided
term reconstruction from ML to Isabelle; for applications which
do not need to be fully trusted.

\item[Property conversion.]  Evaluates propositions; since these
are monomorphic, the term reconstruction is fixed once and for all
and therefore trustable.

\item[Conversion.]  Evaluates an arbitrary term @{term "t"} first
by plain evaluation and certifies the result @{term "t'"} by
checking the equation @{term "t \<equiv> t'"} using property
conversion.

\end{description}

\noindent The picture is further complicated by the roles of
exceptions.  Here three cases have to be distinguished:

\begin{itemize}

\item Evaluation of @{term t} terminates with a result @{term
"t'"}.

\item Evaluation of @{term t} terminates which en exception
indicating a pattern match failure or a non-implemented
function.  As sketched in \secref{sec:partiality}, this can be
interpreted as partiality.

\item Evaluation raises any other kind of exception.

\end{itemize}

\noindent For conversions, the first case yields the equation @{term
"t = t'"}, the second defaults to reflexivity @{term "t = t"}.
Exceptions of the third kind are propagated to the user.

By default return values of plain evaluation are optional, yielding
@{text "SOME t'"} in the first case, @{text "NONE"} in the
second, and propagating the exception in the third case.  A strict
variant of plain evaluation either yields @{text "t'"} or propagates
any exception, a liberal variant caputures any exception in a result
of type @{text "Exn.result"}.

For property conversion (which coincides with conversion except for
evaluation in ML), methods are provided which solve a given goal by
evaluation.
*}

subsection {* Schematic overview *}

text {*
\newcommand{\ttsize}{\fontsize{5.8pt}{8pt}\selectfont}
\fontsize{9pt}{12pt}\selectfont
\begin{tabular}{ll||c|c|c}
& & @{text simp} & @{text nbe} & @{text code} \tabularnewline \hline \hline
\multirow{5}{1ex}{\rotatebox{90}{dynamic}}
& interactive evaluation
& @{command value} @{text "[simp]"} & @{command value} @{text "[nbe]"} & @{command value} @{text "[code]"}
\tabularnewline
& plain evaluation & & & \ttsize@{ML "Code_Evaluation.dynamic_value"} \tabularnewline \cline{2-5}
& evaluation method & @{method code_simp} & @{method normalization} & @{method eval} \tabularnewline
& property conversion & & & \ttsize@{ML "Code_Runtime.dynamic_holds_conv"} \tabularnewline \cline{2-5}
& conversion & \ttsize@{ML "Code_Simp.dynamic_eval_conv"} & \ttsize@{ML "Nbe.dynamic_eval_conv"}
& \ttsize@{ML "Code_Evaluation.dynamic_eval_conv"} \tabularnewline \hline \hline
\multirow{3}{1ex}{\rotatebox{90}{static}}
& plain evaluation & & & \ttsize@{ML "Code_Evaluation.static_value"} \tabularnewline \cline{2-5}
& property conversion & &
& \ttsize@{ML "Code_Runtime.static_holds_conv"} \tabularnewline \cline{2-5}
& conversion & \ttsize@{ML "Code_Simp.static_eval_conv"}
& \ttsize@{ML "Nbe.static_eval_conv"}
& \ttsize@{ML "Code_Evaluation.static_eval_conv"}
\end{tabular}
*}

subsection {* Intimate connection between logic and system runtime *}

text {*
The toolbox of static evaluation conversions forms a reasonable base
to interweave generated code and system tools.  However in some
situations more direct interaction is desirable.
*}

subsubsection {* Static embedding of generated code into system runtime -- the @{text code} antiquotation *}

text {*
The @{text code} antiquotation allows to include constants from
generated code directly into ML system code, as in the following toy
example:
*}

datatype %quote form = T | F | And form form | Or form form (*<*)

(*>*) ML %quotett {*
fun eval_form @{code T} = true
| eval_form @{code F} = false
| eval_form (@{code And} (p, q)) =
eval_form p andalso eval_form q
| eval_form (@{code Or} (p, q)) =
eval_form p orelse eval_form q;
*}

text {*
\noindent @{text code} takes as argument the name of a constant;
after the whole ML is read, the necessary code is generated
transparently and the corresponding constant names are inserted.
This technique also allows to use pattern matching on constructors
stemming from compiled datatypes.  Note that the @{text code}
antiquotation may not refer to constants which carry adaptations;
here you have to refer to the corresponding adapted code directly.

For a less simplistic example, theory @{text Approximation} in
the @{text Decision_Procs} session is a good reference.
*}

subsubsection {* Static embedding of generated code into system runtime -- @{text code_reflect} *}

text {*
The @{text code} antiquoation is lightweight, but the generated code
is only accessible while the ML section is processed.  Sometimes this
is not appropriate, especially if the generated code contains datatype
declarations which are shared with other parts of the system.  In these
cases, @{command_def code_reflect} can be used:
*}

code_reflect %quote Sum_Type
datatypes sum = Inl | Inr
functions "Sum_Type.Projl" "Sum_Type.Projr"

text {*
\noindent @{command_def code_reflect} takes a structure name and
references to datatypes and functions; for these code is compiled
into the named ML structure and the \emph{Eval} target is modified
in a way that future code generation will reference these
precompiled versions of the given datatypes and functions.  This
also allows to refer to the referenced datatypes and functions from
arbitrary ML code as well.

A typical example for @{command code_reflect} can be found in the
@{theory Predicate} theory.
*}

subsubsection {* Separate compilation -- @{text code_reflect} *}

text {*
For technical reasons it is sometimes necessary to separate
generation and compilation of code which is supposed to be used in
the system runtime.  For this @{command code_reflect} with an
optional @{text "file"} argument can be used:
*}

code_reflect %quote Rat
datatypes rat = Frct
functions Fract
"(plus :: rat \<Rightarrow> rat \<Rightarrow> rat)" "(minus :: rat \<Rightarrow> rat \<Rightarrow> rat)"
"(times :: rat \<Rightarrow> rat \<Rightarrow> rat)" "(divide :: rat \<Rightarrow> rat \<Rightarrow> rat)"
file "examples/rat.ML"

text {*
\noindent This merely generates the referenced code to the given
file which can be included into the system runtime later on.
*}

end