theory Introduction
imports Setup
begin
section {* Introduction *}
text {*
This tutorial introduces the code generator facilities of @{text
"Isabelle/HOL"}. It allows to turn (a certain class of) HOL
specifications into corresponding executable code in the programming
languages @{text SML} \cite{SML}, @{text OCaml} \cite{OCaml} and
@{text Haskell} \cite{haskell-revised-report}.
To profit from this tutorial, some familiarity and experience with
@{theory HOL} \cite{isa-tutorial} and its basic theories is assumed.
*}
subsection {* Code generation principle: shallow embedding \label{sec:principle} *}
text {*
The key concept for understanding Isabelle's code generation is
\emph{shallow embedding}: logical entities like constants, types and
classes are identified with corresponding entities in the target
language. In particular, the carrier of a generated program's
semantics are \emph{equational theorems} from the logic. If we view
a generated program as an implementation of a higher-order rewrite
system, then every rewrite step performed by the program can be
simulated in the logic, which guarantees partial correctness
\cite{Haftmann-Nipkow:2010:code}.
*}
subsection {* A quick start with the Isabelle/HOL toolbox \label{sec:queue_example} *}
text {*
In a HOL theory, the @{command datatype} and @{command
definition}/@{command primrec}/@{command fun} declarations form the
core of a functional programming language. By default equational
theorems stemming from those are used for generated code, therefore
\qt{naive} code generation can proceed without further ado.
For example, here a simple \qt{implementation} of amortised queues:
*}
datatype %quote 'a queue = AQueue "'a list" "'a list"
definition %quote empty :: "'a queue" where
"empty = AQueue [] []"
primrec %quote enqueue :: "'a \<Rightarrow> 'a queue \<Rightarrow> 'a queue" where
"enqueue x (AQueue xs ys) = AQueue (x # xs) ys"
fun %quote dequeue :: "'a queue \<Rightarrow> 'a option \<times> 'a queue" where
"dequeue (AQueue [] []) = (None, AQueue [] [])"
| "dequeue (AQueue xs (y # ys)) = (Some y, AQueue xs ys)"
| "dequeue (AQueue xs []) =
(case rev xs of y # ys \<Rightarrow> (Some y, AQueue [] ys))" (*<*)
lemma %invisible dequeue_nonempty_Nil [simp]:
"xs \<noteq> [] \<Longrightarrow> dequeue (AQueue xs []) = (case rev xs of y # ys \<Rightarrow> (Some y, AQueue [] ys))"
by (cases xs) (simp_all split: list.splits) (*>*)
text {* \noindent Then we can generate code e.g.~for @{text SML} as follows: *}
export_code %quote empty dequeue enqueue in SML
module_name Example file "examples/example.ML"
text {* \noindent resulting in the following code: *}
text %quote {*@{code_stmts empty enqueue dequeue (SML)}*}
text {*
\noindent The @{command export_code} command takes a space-separated
list of constants for which code shall be generated; anything else
needed for those is added implicitly. Then follows a target
language identifier and a freely chosen module name. A file name
denotes the destination to store the generated code. Note that the
semantics of the destination depends on the target language: for
@{text SML} and @{text OCaml} it denotes a \emph{file}, for @{text
Haskell} it denotes a \emph{directory} where a file named as the
module name (with extension @{text ".hs"}) is written:
*}
export_code %quote empty dequeue enqueue in Haskell
module_name Example file "examples/"
text {*
\noindent This is the corresponding code:
*}
text %quote {*@{code_stmts empty enqueue dequeue (Haskell)}*}
text {*
\noindent For more details about @{command export_code} see
\secref{sec:further}.
*}
subsection {* Type classes *}
text {*
Code can also be generated from type classes in a Haskell-like
manner. For illustration here an example from abstract algebra:
*}
class %quote semigroup =
fixes mult :: "'a \<Rightarrow> 'a \<Rightarrow> 'a" (infixl "\<otimes>" 70)
assumes assoc: "(x \<otimes> y) \<otimes> z = x \<otimes> (y \<otimes> z)"
class %quote monoid = semigroup +
fixes neutral :: 'a ("\<one>")
assumes neutl: "\<one> \<otimes> x = x"
and neutr: "x \<otimes> \<one> = x"
instantiation %quote nat :: monoid
begin
primrec %quote mult_nat where
"0 \<otimes> n = (0\<Colon>nat)"
| "Suc m \<otimes> n = n + m \<otimes> n"
definition %quote neutral_nat where
"\<one> = Suc 0"
lemma %quote add_mult_distrib:
fixes n m q :: nat
shows "(n + m) \<otimes> q = n \<otimes> q + m \<otimes> q"
by (induct n) simp_all
instance %quote proof
fix m n q :: nat
show "m \<otimes> n \<otimes> q = m \<otimes> (n \<otimes> q)"
by (induct m) (simp_all add: add_mult_distrib)
show "\<one> \<otimes> n = n"
by (simp add: neutral_nat_def)
show "m \<otimes> \<one> = m"
by (induct m) (simp_all add: neutral_nat_def)
qed
end %quote
text {*
\noindent We define the natural operation of the natural numbers
on monoids:
*}
primrec %quote (in monoid) pow :: "nat \<Rightarrow> 'a \<Rightarrow> 'a" where
"pow 0 a = \<one>"
| "pow (Suc n) a = a \<otimes> pow n a"
text {*
\noindent This we use to define the discrete exponentiation
function:
*}
definition %quote bexp :: "nat \<Rightarrow> nat" where
"bexp n = pow n (Suc (Suc 0))"
text {*
\noindent The corresponding code in Haskell uses that language's
native classes:
*}
text %quote {*@{code_stmts bexp (Haskell)}*}
text {*
\noindent This is a convenient place to show how explicit dictionary
construction manifests in generated code -- the same example in
@{text SML}:
*}
text %quote {*@{code_stmts bexp (SML)}*}
text {*
\noindent Note the parameters with trailing underscore (@{verbatim
"A_"}), which are the dictionary parameters.
*}
subsection {* How to continue from here *}
text {*
What you have seen so far should be already enough in a lot of
cases. If you are content with this, you can quit reading here.
Anyway, to understand situations where problems occur or to increase
the scope of code generation beyond default, it is necessary to gain
some understanding how the code generator actually works:
\begin{itemize}
\item The foundations of the code generator are described in
\secref{sec:foundations}.
\item In particular \secref{sec:utterly_wrong} gives hints how to
debug situations where code generation does not succeed as
expected.
\item The scope and quality of generated code can be increased
dramatically by applying refinement techniques, which are
introduced in \secref{sec:refinement}.
\item Inductive predicates can be turned executable using an
extension of the code generator \secref{sec:inductive}.
\item You may want to skim over the more technical sections
\secref{sec:adaptation} and \secref{sec:further}.
\item For exhaustive syntax diagrams etc. you should visit the
Isabelle/Isar Reference Manual \cite{isabelle-isar-ref}.
\end{itemize}
\bigskip
\begin{center}\fbox{\fbox{\begin{minipage}{8cm}
\begin{center}\textit{Happy proving, happy hacking!}\end{center}
\end{minipage}}}\end{center}
\begin{warn}
There is also a more ancient code generator in Isabelle by Stefan
Berghofer \cite{Berghofer-Nipkow:2002}. Although its
functionality is covered by the code generator presented here, it
will sometimes show up as an artifact. In case of ambiguity, we
will refer to the framework described here as @{text "generic code
generator"}, to the other as @{text "SML code generator"}.
\end{warn}
*}
end