doc-src/IsarAdvanced/Codegen/Thy/Adaption.thy
author haftmann
Wed Oct 01 13:33:54 2008 +0200 (2008-10-01)
changeset 28447 df77ed974a78
parent 28428 fd007794561f
child 28456 7a558c872104
permissions -rw-r--r--
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theory Adaption
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imports Setup
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begin
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section {* Adaption to target languages \label{sec:adaption} *}
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subsection {* Common adaption cases *}
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text {*
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  The @{theory HOL} @{theory Main} theory already provides a code
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  generator setup
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  which should be suitable for most applications. Common extensions
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  and modifications are available by certain theories of the @{text HOL}
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  library; beside being useful in applications, they may serve
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  as a tutorial for customising the code generator setup (see below
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  \secref{sec:adaption_mechanisms}).
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  \begin{description}
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    \item[@{theory "Code_Integer"}] represents @{text HOL} integers by big
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       integer literals in target languages.
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    \item[@{theory "Code_Char"}] represents @{text HOL} characters by 
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       character literals in target languages.
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    \item[@{theory "Code_Char_chr"}] like @{text "Code_Char"},
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       but also offers treatment of character codes; includes
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       @{theory "Code_Char_chr"}.
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    \item[@{theory "Efficient_Nat"}] \label{eff_nat} implements natural numbers by integers,
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       which in general will result in higher efficiency; pattern
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       matching with @{term "0\<Colon>nat"} / @{const "Suc"}
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       is eliminated;  includes @{theory "Code_Integer"}.
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    \item[@{theory "Code_Index"}] provides an additional datatype
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       @{typ index} which is mapped to target-language built-in integers.
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       Useful for code setups which involve e.g. indexing of
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       target-language arrays.
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    \item[@{theory "Code_Message"}] provides an additional datatype
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       @{typ message_string} which is isomorphic to strings;
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       @{typ message_string}s are mapped to target-language strings.
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       Useful for code setups which involve e.g. printing (error) messages.
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  \end{description}
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  \begin{warn}
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    When importing any of these theories, they should form the last
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    items in an import list.  Since these theories adapt the
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    code generator setup in a non-conservative fashion,
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    strange effects may occur otherwise.
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  \end{warn}
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*}
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subsection {* Adaption mechanisms \label{sec:adaption_mechanisms} *}
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text {*
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  \begin{warn}
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    The mechanisms shown here are especially for the curious;  the user
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    rarely needs to do anything on his own beyond the defaults in @{text HOL}.
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    Adaption is a delicated task which requires a lot of dilligence since
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    it happend \emph{completely} outside the logic.
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  \end{warn}
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*}
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text {*
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  Consider the following function and its corresponding
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  SML code:
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*}
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primrec %quoteme in_interval :: "nat \<times> nat \<Rightarrow> nat \<Rightarrow> bool" where
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  "in_interval (k, l) n \<longleftrightarrow> k \<le> n \<and> n \<le> l"
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(*<*)
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code_type %invisible bool
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  (SML)
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code_const %invisible True and False and "op \<and>" and Not
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  (SML and and and)
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(*>*)
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text %quoteme {*@{code_stmts in_interval (SML)}*}
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text {*
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  \noindent Though this is correct code, it is a little bit unsatisfactory:
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  boolean values and operators are materialised as distinguished
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  entities with have nothing to do with the SML-built-in notion
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  of \qt{bool}.  This results in less readable code;
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  additionally, eager evaluation may cause programs to
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  loop or break which would perfectly terminate when
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  the existing SML @{verbatim "bool"} would be used.  To map
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  the HOL @{typ bool} on SML @{verbatim "bool"}, we may use
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  \qn{custom serialisations}:
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*}
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code_type %tt bool
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  (SML "bool")
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code_const %tt True and False and "op \<and>"
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  (SML "true" and "false" and "_ andalso _")
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text {*
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  \noindent The @{command code_type} command takes a type constructor
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  as arguments together with a list of custom serialisations.
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  Each custom serialisation starts with a target language
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  identifier followed by an expression, which during
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  code serialisation is inserted whenever the type constructor
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  would occur.  For constants, @{command code_const} implements
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  the corresponding mechanism.  Each ``@{verbatim "_"}'' in
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  a serialisation expression is treated as a placeholder
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  for the type constructor's (the constant's) arguments.
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*}
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text %quoteme {*@{code_stmts in_interval (SML)}*}
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text {*
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  \noindent This still is not perfect: the parentheses
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  around the \qt{andalso} expression are superfluous.
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  Though the serializer
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  by no means attempts to imitate the rich Isabelle syntax
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  framework, it provides some common idioms, notably
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  associative infixes with precedences which may be used here:
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*}
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code_const %tt "op \<and>"
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  (SML infixl 1 "andalso")
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text %quoteme {*@{code_stmts in_interval (SML)}*}
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text {*
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  \noindent Next, we try to map HOL pairs to SML pairs, using the
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  infix ``@{verbatim "*"}'' type constructor and parentheses:
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*}
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(*<*)
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code_type %invisible *
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  (SML)
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code_const %invisible Pair
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  (SML)
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(*>*)
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code_type %tt *
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  (SML infix 2 "*")
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code_const %tt Pair
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  (SML "!((_),/ (_))")
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text {*
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  \noindent The initial bang ``@{verbatim "!"}'' tells the serializer to never put
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  parentheses around the whole expression (they are already present),
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  while the parentheses around argument place holders
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  tell not to put parentheses around the arguments.
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  The slash ``@{verbatim "/"}'' (followed by arbitrary white space)
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  inserts a space which may be used as a break if necessary
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  during pretty printing.
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  These examples give a glimpse what mechanisms
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  custom serialisations provide; however their usage
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  requires careful thinking in order not to introduce
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  inconsistencies -- or, in other words:
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  custom serialisations are completely axiomatic.
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  A further noteworthy details is that any special
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  character in a custom serialisation may be quoted
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  using ``@{verbatim "'"}''; thus, in
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  ``@{verbatim "fn '_ => _"}'' the first
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  ``@{verbatim "_"}'' is a proper underscore while the
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  second ``@{verbatim "_"}'' is a placeholder.
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  The HOL theories provide further
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  examples for custom serialisations.
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*}
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subsection {* @{text Haskell} serialisation *}
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text {*
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  For convenience, the default
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  @{text HOL} setup for @{text Haskell} maps the @{class eq} class to
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  its counterpart in @{text Haskell}, giving custom serialisations
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  for the class @{class eq} (by command @{command code_class}) and its operation
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  @{const HOL.eq}
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*}
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code_class %tt eq
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  (Haskell "Eq" where "HOL.eq" \<equiv> "(==)")
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code_const %tt "op ="
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  (Haskell infixl 4 "==")
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text {*
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  \noindent A problem now occurs whenever a type which
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  is an instance of @{class eq} in @{text HOL} is mapped
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  on a @{text Haskell}-built-in type which is also an instance
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  of @{text Haskell} @{text Eq}:
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*}
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typedecl %quoteme bar
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instantiation %quoteme bar :: eq
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begin
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definition %quoteme "eq_class.eq (x\<Colon>bar) y \<longleftrightarrow> x = y"
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instance %quoteme by default (simp add: eq_bar_def)
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end %quoteme
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code_type %tt bar
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  (Haskell "Integer")
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text {*
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  \noindent The code generator would produce
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  an additional instance, which of course is rejectedby the @{text Haskell}
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  compiler.
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  To suppress this additional instance, use
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  @{text "code_instance"}:
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*}
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code_instance %tt bar :: eq
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  (Haskell -)
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end