src/Doc/Codegen/Further.thy

+theory Further
+imports Setup
+begin
+
+section {* Further issues \label{sec:further} *}
+
+subsection {* Specialities of the @{text Scala} target language \label{sec:scala} *}
+
+text {*
+  @{text Scala} deviates from languages of the ML family in a couple
+  of aspects; those which affect code generation mainly have to do with
+  @{text Scala}'s type system:
+
+  \begin{itemize}
+
+    \item @{text Scala} prefers tupled syntax over curried syntax.
+
+    \item @{text Scala} sacrifices Hindely-Milner type inference for a
+      much more rich type system with subtyping etc.  For this reason
+      type arguments sometimes have to be given explicitly in square
+      brackets (mimicking System F syntax).
+
+    \item In contrast to @{text Haskell} where most specialities of
+      the type system are implemented using \emph{type classes},
+      @{text Scala} provides a sophisticated system of \emph{implicit
+      arguments}.
+
+  \end{itemize}
+
+  \noindent Concerning currying, the @{text Scala} serializer counts
+  arguments in code equations to determine how many arguments
+  shall be tupled; remaining arguments and abstractions in terms
+  rather than function definitions are always curried.
+
+  The second aspect affects user-defined adaptations with @{command
+  code_const}.  For regular terms, the @{text Scala} serializer prints
+  all type arguments explicitly.  For user-defined term adaptations
+  this is only possible for adaptations which take no arguments: here
+  the type arguments are just appended.  Otherwise they are ignored;
+  hence user-defined adaptations for polymorphic constants have to be
+  designed very carefully to avoid ambiguity.
+
+  Isabelle's type classes are mapped onto @{text Scala} implicits; in
+  cases with diamonds in the subclass hierarchy this can lead to
+  ambiguities in the generated code:
+*}
+
+class %quote class1 =
+  fixes foo :: "'a \<Rightarrow> 'a"
+
+class %quote class2 = class1
+
+class %quote class3 = class1
+
+text {*
+  \noindent Here both @{class class2} and @{class class3} inherit from @{class class1},
+  forming the upper part of a diamond.
+*}
+
+definition %quote bar :: "'a :: {class2, class3} \<Rightarrow> 'a" where
+  "bar = foo"
+
+text {*
+  \noindent This yields the following code:
+*}
+
+text %quotetypewriter {*
+  @{code_stmts bar (Scala)}
+*}
+
+text {*
+  \noindent This code is rejected by the @{text Scala} compiler: in
+  the definition of @{text bar}, it is not clear from where to derive
+  the implicit argument for @{text foo}.
+
+  The solution to the problem is to close the diamond by a further
+  class with inherits from both @{class class2} and @{class class3}:
+*}
+
+class %quote class4 = class2 + class3
+
+text {*
+  \noindent Then the offending code equation can be restricted to
+  @{class class4}:
+*}
+
+lemma %quote [code]:
+  "(bar :: 'a::class4 \<Rightarrow> 'a) = foo"
+  by (simp only: bar_def)
+
+text {*
+  \noindent with the following code:
+*}
+
+text %quotetypewriter {*
+  @{code_stmts bar (Scala)}
+*}
+
+text {*
+  \noindent which exposes no ambiguity.
+
+  Since the preprocessor (cf.~\secref{sec:preproc}) propagates sort
+  constraints through a system of code equations, it is usually not
+  very difficult to identify the set of code equations which actually
+  needs more restricted sort constraints.
+*}
+
+subsection {* Modules namespace *}
+
+text {*
+  When invoking the @{command export_code} command it is possible to
+  leave out the @{keyword "module_name"} part; then code is
+  distributed over different modules, where the module name space
+  roughly is induced by the Isabelle theory name space.
+
+  Then sometimes the awkward situation occurs that dependencies
+  between definitions introduce cyclic dependencies between modules,
+  which in the @{text Haskell} world leaves you to the mercy of the
+  @{text Haskell} implementation you are using, while for @{text
+  SML}/@{text OCaml} code generation is not possible.
+
+  A solution is to declare module names explicitly.  Let use assume
+  the three cyclically dependent modules are named \emph{A}, \emph{B}
+  and \emph{C}.  Then, by stating
+*}
+
+code_modulename %quote SML
+  A ABC
+  B ABC
+  C ABC
+
+text {*
+  \noindent we explicitly map all those modules on \emph{ABC},
+  resulting in an ad-hoc merge of this three modules at serialisation
+  time.
+*}
+
+subsection {* Locales and interpretation *}
+
+text {*
+  A technical issue comes to surface when generating code from
+  specifications stemming from locale interpretation.
+
+  Let us assume a locale specifying a power operation on arbitrary
+  types:
+*}
+
+locale %quote power =
+  fixes power :: "'a \<Rightarrow> 'b \<Rightarrow> 'b"
+  assumes power_commute: "power x \<circ> power y = power y \<circ> power x"
+begin
+
+text {*
+  \noindent Inside that locale we can lift @{text power} to exponent
+  lists by means of specification relative to that locale:
+*}
+
+primrec %quote powers :: "'a list \<Rightarrow> 'b \<Rightarrow> 'b" where
+  "powers [] = id"
+| "powers (x # xs) = power x \<circ> powers xs"
+
+lemma %quote powers_append:
+  "powers (xs @ ys) = powers xs \<circ> powers ys"
+  by (induct xs) simp_all
+
+lemma %quote powers_power:
+  "powers xs \<circ> power x = power x \<circ> powers xs"
+  by (induct xs)
+    (simp_all del: o_apply id_apply add: o_assoc [symmetric],
+      simp del: o_apply add: o_assoc power_commute)
+
+lemma %quote powers_rev:
+  "powers (rev xs) = powers xs"
+    by (induct xs) (simp_all add: powers_append powers_power)
+
+end %quote
+
+text {*
+  After an interpretation of this locale (say, @{command_def
+  interpretation} @{text "fun_power:"} @{term [source] "power (\<lambda>n (f
+  :: 'a \<Rightarrow> 'a). f ^^ n)"}), one would expect to have a constant @{text
+  "fun_power.powers :: nat list \<Rightarrow> ('a \<Rightarrow> 'a) \<Rightarrow> 'a \<Rightarrow> 'a"} for which code
+  can be generated.  But this not the case: internally, the term
+  @{text "fun_power.powers"} is an abbreviation for the foundational
+  term @{term [source] "power.powers (\<lambda>n (f :: 'a \<Rightarrow> 'a). f ^^ n)"}
+  (see \cite{isabelle-locale} for the details behind).
+
+  Fortunately, with minor effort the desired behaviour can be
+  achieved.  First, a dedicated definition of the constant on which
+  the local @{text "powers"} after interpretation is supposed to be
+  mapped on:
+*}
+
+definition %quote funpows :: "nat list \<Rightarrow> ('a \<Rightarrow> 'a) \<Rightarrow> 'a \<Rightarrow> 'a" where
+  [code del]: "funpows = power.powers (\<lambda>n f. f ^^ n)"
+
+text {*
+  \noindent In general, the pattern is @{text "c = t"} where @{text c}
+  is the name of the future constant and @{text t} the foundational
+  term corresponding to the local constant after interpretation.
+
+  The interpretation itself is enriched with an equation @{text "t = c"}:
+*}
+
+interpretation %quote fun_power: power "\<lambda>n (f :: 'a \<Rightarrow> 'a). f ^^ n" where
+  "power.powers (\<lambda>n f. f ^^ n) = funpows"
+  by unfold_locales
+    (simp_all add: fun_eq_iff funpow_mult mult_commute funpows_def)
+
+text {*
+  \noindent This additional equation is trivially proved by the
+  definition itself.
+
+  After this setup procedure, code generation can continue as usual:
+*}
+
+text %quotetypewriter {*
+  @{code_stmts funpows (consts) Nat.funpow funpows (Haskell)}
+*}
+
+
+subsection {* Imperative data structures *}
+
+text {*
+  If you consider imperative data structures as inevitable for a
+  specific application, you should consider \emph{Imperative
+  Functional Programming with Isabelle/HOL}
+  \cite{bulwahn-et-al:2008:imperative}; the framework described there
+  is available in session @{text Imperative_HOL}, together with a
+  short primer document.
+*}
+
+
+subsection {* ML system interfaces \label{sec:ml} *}
+
+text {*
+  Since the code generator framework not only aims to provide a nice
+  Isar interface but also to form a base for code-generation-based
+  applications, here a short description of the most fundamental ML
+  interfaces.
+*}
+
+subsubsection {* Managing executable content *}
+
+text %mlref {*
+  \begin{mldecls}
+  @{index_ML Code.read_const: "theory -> string -> string"} \\
+  @{index_ML Code.add_eqn: "thm -> theory -> theory"} \\
+  @{index_ML Code.del_eqn: "thm -> theory -> theory"} \\
+  @{index_ML Code_Preproc.map_pre: "(simpset -> simpset) -> theory -> theory"} \\
+  @{index_ML Code_Preproc.map_post: "(simpset -> simpset) -> theory -> theory"} \\
+  @{index_ML Code_Preproc.add_functrans: "
+    string * (theory -> (thm * bool) list -> (thm * bool) list option)
+      -> theory -> theory"} \\
+  @{index_ML Code_Preproc.del_functrans: "string -> theory -> theory"} \\
+  @{index_ML Code.add_datatype: "(string * typ) list -> theory -> theory"} \\
+  @{index_ML Code.get_type: "theory -> string
+    -> ((string * sort) list * (string * ((string * sort) list * typ list)) list) * bool"} \\
+  @{index_ML Code.get_type_of_constr_or_abstr: "theory -> string -> (string * bool) option"}
+  \end{mldecls}
+
+  \begin{description}
+
+  \item @{ML Code.read_const}~@{text thy}~@{text s}
+     reads a constant as a concrete term expression @{text s}.
+
+  \item @{ML Code.add_eqn}~@{text "thm"}~@{text "thy"} adds function
+     theorem @{text "thm"} to executable content.
+
+  \item @{ML Code.del_eqn}~@{text "thm"}~@{text "thy"} removes function
+     theorem @{text "thm"} from executable content, if present.
+
+  \item @{ML Code_Preproc.map_pre}~@{text "f"}~@{text "thy"} changes
+     the preprocessor simpset.
+
+  \item @{ML Code_Preproc.add_functrans}~@{text "(name, f)"}~@{text "thy"} adds
+     function transformer @{text f} (named @{text name}) to executable content;
+     @{text f} is a transformer of the code equations belonging
+     to a certain function definition, depending on the
+     current theory context.  Returning @{text NONE} indicates that no
+     transformation took place;  otherwise, the whole process will be iterated
+     with the new code equations.
+
+  \item @{ML Code_Preproc.del_functrans}~@{text "name"}~@{text "thy"} removes
+     function transformer named @{text name} from executable content.
+
+  \item @{ML Code.add_datatype}~@{text cs}~@{text thy} adds
+     a datatype to executable content, with generation
+     set @{text cs}.
+
+  \item @{ML Code.get_type_of_constr_or_abstr}~@{text "thy"}~@{text "const"}
+     returns type constructor corresponding to
+     constructor @{text const}; returns @{text NONE}
+     if @{text const} is no constructor.
+
+  \end{description}
+*}
+
+
+subsubsection {* Data depending on the theory's executable content *}
+
+text {*
+  Implementing code generator applications on top of the framework set
+  out so far usually not only involves using those primitive
+  interfaces but also storing code-dependent data and various other
+  things.
+
+  Due to incrementality of code generation, changes in the theory's
+  executable content have to be propagated in a certain fashion.
+  Additionally, such changes may occur not only during theory
+  extension but also during theory merge, which is a little bit nasty
+  from an implementation point of view.  The framework provides a
+  solution to this technical challenge by providing a functorial data
+  slot @{ML_functor Code_Data}; on instantiation of this functor, the
+  following types and operations are required:
+
+  \medskip
+  \begin{tabular}{l}
+  @{text "type T"} \\
+  @{text "val empty: T"} \\
+  \end{tabular}
+
+  \begin{description}
+
+  \item @{text T} the type of data to store.
+
+  \item @{text empty} initial (empty) data.
+
+  \end{description}
+
+  \noindent An instance of @{ML_functor Code_Data} provides the
+  following interface:
+
+  \medskip
+  \begin{tabular}{l}
+  @{text "change: theory \<rightarrow> (T \<rightarrow> T) \<rightarrow> T"} \\
+  @{text "change_yield: theory \<rightarrow> (T \<rightarrow> 'a * T) \<rightarrow> 'a * T"}
+  \end{tabular}
+
+  \begin{description}
+
+  \item @{text change} update of current data (cached!) by giving a
+    continuation.
+
+  \item @{text change_yield} update with side result.
+
+  \end{description}
+*}
+
+end
+