theory Introduction
imports Setup
begin (*<*)
ML \<open>
Isabelle_System.make_directory (File.tmp_path (Path.basic "examples"))
\<close> (*>*)
section \<open>Introduction\<close>
text \<open>
This tutorial introduces the code generator facilities of \<open>Isabelle/HOL\<close>. It allows to turn (a certain class of) HOL
specifications into corresponding executable code in the programming
languages \<open>SML\<close> @{cite SML}, \<open>OCaml\<close> @{cite OCaml},
\<open>Haskell\<close> @{cite "haskell-revised-report"} and \<open>Scala\<close>
@{cite "scala-overview-tech-report"}.
To profit from this tutorial, some familiarity and experience with
Isabelle/HOL @{cite "isa-tutorial"} and its basic theories is assumed.
\<close>
subsection \<open>Code generation principle: shallow embedding \label{sec:principle}\<close>
text \<open>
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"}.
\<close>
subsection \<open>A quick start with the Isabelle/HOL toolbox \label{sec:queue_example}\<close>
text \<open>
In a HOL theory, the @{command_def datatype} and @{command_def
definition}/@{command_def primrec}/@{command_def 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:
\<close>
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 \<open>\noindent Then we can generate code e.g.~for \<open>SML\<close> as follows:\<close>
export_code %quote empty dequeue enqueue in SML module_name Example
text \<open>\noindent resulting in the following code:\<close>
text %quote \<open>
@{code_stmts empty enqueue dequeue (SML)}
\<close>
text \<open>
\noindent The @{command_def export_code} command takes multiple 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 \<^theory_text>\<open>module_name\<close>.
Output is written to a logical file-system within the theory context,
with the theory name and ``\<^verbatim>\<open>code\<close>'' as overall prefix. There is also a
formal session export using the same name: the result may be explored in
the Isabelle/jEdit Prover IDE using the file-browser on the URL
``\<^verbatim>\<open>isabelle-export:\<close>''.
The file name is determined by the target language together with an
optional \<^theory_text>\<open>file_prefix\<close> (the default is ``\<^verbatim>\<open>export\<close>'' with a consecutive
number within the current theory). For \<open>SML\<close>, \<open>OCaml\<close> and \<open>Scala\<close>, the
file prefix becomes a plain file with extension (e.g.\ ``\<^verbatim>\<open>.ML\<close>'' for
SML). For \<open>Haskell\<close> the file prefix becomes a directory that is populated
with a separate file for each module (with extension ``\<^verbatim>\<open>.hs\<close>'').
Consider the following example:
\<close>
export_code %quote empty dequeue enqueue in Haskell
module_name Example file_prefix example
text \<open>
\noindent This is the corresponding code:
\<close>
text %quote \<open>
@{code_stmts empty enqueue dequeue (Haskell)}
\<close>
text \<open>
\noindent For more details about @{command export_code} see
\secref{sec:further}.
\<close>
subsection \<open>Type classes\<close>
text \<open>
Code can also be generated from type classes in a Haskell-like
manner. For illustration here an example from abstract algebra:
\<close>
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::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 \<open>
\noindent We define the natural operation of the natural numbers
on monoids:
\<close>
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 \<open>
\noindent This we use to define the discrete exponentiation
function:
\<close>
definition %quote bexp :: "nat \<Rightarrow> nat" where
"bexp n = pow n (Suc (Suc 0))"
text \<open>
\noindent The corresponding code in Haskell uses that language's
native classes:
\<close>
text %quote \<open>
@{code_stmts bexp (Haskell)}
\<close>
text \<open>
\noindent This is a convenient place to show how explicit dictionary
construction manifests in generated code -- the same example in
\<open>SML\<close>:
\<close>
text %quote \<open>
@{code_stmts bexp (SML)}
\<close>
text \<open>
\noindent Note the parameters with trailing underscore (\<^verbatim>\<open>A_\<close>), which are the dictionary parameters.
\<close>
subsection \<open>How to continue from here\<close>
text \<open>
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 If you want to utilize code generation to obtain fast
evaluators e.g.~for decision procedures, have a look at
\secref{sec:evaluation}.
\item You may want to skim over the more technical sections
\secref{sec:adaptation} and \secref{sec:further}.
\item The target language Scala @{cite "scala-overview-tech-report"}
comes with some specialities discussed in \secref{sec:scala}.
\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}
\<close>
end