doc-src/Codegen/Thy/Adaptation.thy
 author haftmann Thu Sep 23 15:46:17 2010 +0200 (2010-09-23) changeset 39664 0afaf89ab591 parent 39643 29cc021398fc child 39683 f75a01ee6c41 permissions -rw-r--r--
more canonical type setting of type writer code examples
     1 theory Adaptation

     2 imports Setup

     3 begin

     4

     5 setup %invisible {* Code_Target.extend_target ("\<SML>", ("SML", K I)) *}

     6

     7 section {* Adaptation to target languages \label{sec:adaptation} *}

     8

     9 subsection {* Adapting code generation *}

    10

    11 text {*

    12   The aspects of code generation introduced so far have two aspects

    13   in common:

    14

    15   \begin{itemize}

    16

    17     \item They act uniformly, without reference to a specific target

    18        language.

    19

    20     \item They are \emph{safe} in the sense that as long as you trust

    21        the code generator meta theory and implementation, you cannot

    22        produce programs that yield results which are not derivable in

    23        the logic.

    24

    25   \end{itemize}

    26

    27   \noindent In this section we will introduce means to \emph{adapt}

    28   the serialiser to a specific target language, i.e.~to print program

    29   fragments in a way which accommodates \qt{already existing}

    30   ingredients of a target language environment, for three reasons:

    31

    32   \begin{itemize}

    33     \item improving readability and aesthetics of generated code

    34     \item gaining efficiency

    35     \item interface with language parts which have no direct counterpart

    36       in @{text "HOL"} (say, imperative data structures)

    37   \end{itemize}

    38

    39   \noindent Generally, you should avoid using those features yourself

    40   \emph{at any cost}:

    41

    42   \begin{itemize}

    43

    44     \item The safe configuration methods act uniformly on every target

    45       language, whereas for adaptation you have to treat each target

    46       language separately.

    47

    48     \item Application is extremely tedious since there is no

    49       abstraction which would allow for a static check, making it easy

    50       to produce garbage.

    51

    52     \item Subtle errors can be introduced unconsciously.

    53

    54   \end{itemize}

    55

    56   \noindent However, even if you ought refrain from setting up

    57   adaptation yourself, already the @{text "HOL"} comes with some

    58   reasonable default adaptations (say, using target language list

    59   syntax).  There also some common adaptation cases which you can

    60   setup by importing particular library theories.  In order to

    61   understand these, we provide some clues here; these however are not

    62   supposed to replace a careful study of the sources.

    63 *}

    64

    65

    66 subsection {* The adaptation principle *}

    67

    68 text {*

    69   Figure \ref{fig:adaptation} illustrates what \qt{adaptation} is

    70   conceptually supposed to be:

    71

    72   \begin{figure}[here]

    73     \includegraphics{adaptation}

    74     \caption{The adaptation principle}

    75     \label{fig:adaptation}

    76   \end{figure}

    77

    78   \noindent In the tame view, code generation acts as broker between

    79   @{text logic}, @{text "intermediate language"} and @{text "target

    80   language"} by means of @{text translation} and @{text

    81   serialisation}; for the latter, the serialiser has to observe the

    82   structure of the @{text language} itself plus some @{text reserved}

    83   keywords which have to be avoided for generated code.  However, if

    84   you consider @{text adaptation} mechanisms, the code generated by

    85   the serializer is just the tip of the iceberg:

    86

    87   \begin{itemize}

    88

    89     \item @{text serialisation} can be \emph{parametrised} such that

    90       logical entities are mapped to target-specific ones

    91       (e.g. target-specific list syntax, see also

    92       \secref{sec:adaptation_mechanisms})

    93

    94     \item Such parametrisations can involve references to a

    95       target-specific standard @{text library} (e.g. using the @{text

    96       Haskell} @{verbatim Maybe} type instead of the @{text HOL}

    97       @{type "option"} type); if such are used, the corresponding

    98       identifiers (in our example, @{verbatim Maybe}, @{verbatim

    99       Nothing} and @{verbatim Just}) also have to be considered @{text

   100       reserved}.

   101

   102     \item Even more, the user can enrich the library of the

   103       target-language by providing code snippets (\qt{@{text

   104       "includes"}}) which are prepended to any generated code (see

   105       \secref{sec:include}); this typically also involves further

   106       @{text reserved} identifiers.

   107

   108   \end{itemize}

   109

   110   \noindent As figure \ref{fig:adaptation} illustrates, all these

   111   adaptation mechanisms have to act consistently; it is at the

   112   discretion of the user to take care for this.

   113 *}

   114

   115 subsection {* Common adaptation patterns *}

   116

   117 text {*

   118   The @{theory HOL} @{theory Main} theory already provides a code

   119   generator setup which should be suitable for most applications.

   120   Common extensions and modifications are available by certain

   121   theories of the @{text HOL} library; beside being useful in

   122   applications, they may serve as a tutorial for customising the code

   123   generator setup (see below \secref{sec:adaptation_mechanisms}).

   124

   125   \begin{description}

   126

   127     \item[@{theory "Code_Integer"}] represents @{text HOL} integers by

   128        big integer literals in target languages.

   129

   130     \item[@{theory "Code_Char"}] represents @{text HOL} characters by

   131        character literals in target languages.

   132

   133     \item[@{theory "Code_Char_chr"}] like @{text "Code_Char"}, but

   134        also offers treatment of character codes; includes @{theory

   135        "Code_Char"}.

   136

   137     \item[@{theory "Efficient_Nat"}] \label{eff_nat} implements

   138        natural numbers by integers, which in general will result in

   139        higher efficiency; pattern matching with @{term "0\<Colon>nat"} /

   140        @{const "Suc"} is eliminated; includes @{theory "Code_Integer"}

   141        and @{theory "Code_Numeral"}.

   142

   143     \item[@{theory "Code_Numeral"}] provides an additional datatype

   144        @{typ index} which is mapped to target-language built-in

   145        integers.  Useful for code setups which involve e.g.~indexing

   146        of target-language arrays.

   147

   148     \item[@{theory "String"}] provides an additional datatype @{typ

   149        String.literal} which is isomorphic to strings; @{typ

   150        String.literal}s are mapped to target-language strings.  Useful

   151        for code setups which involve e.g.~printing (error) messages.

   152

   153   \end{description}

   154

   155   \begin{warn}

   156     When importing any of these theories, they should form the last

   157     items in an import list.  Since these theories adapt the code

   158     generator setup in a non-conservative fashion, strange effects may

   159     occur otherwise.

   160   \end{warn}

   161 *}

   162

   163

   164 subsection {* Parametrising serialisation \label{sec:adaptation_mechanisms} *}

   165

   166 text {*

   167   Consider the following function and its corresponding SML code:

   168 *}

   169

   170 primrec %quote in_interval :: "nat \<times> nat \<Rightarrow> nat \<Rightarrow> bool" where

   171   "in_interval (k, l) n \<longleftrightarrow> k \<le> n \<and> n \<le> l"

   172 (*<*)

   173 code_type %invisible bool

   174   (SML)

   175 code_const %invisible True and False and "op \<and>" and Not

   176   (SML and and and)

   177 (*>*)

   178 text %quote {*

   179   \begin{typewriter}

   180     @{code_stmts in_interval (SML)}

   181   \end{typewriter}

   182 *}

   183

   184 text {*

   185   \noindent Though this is correct code, it is a little bit

   186   unsatisfactory: boolean values and operators are materialised as

   187   distinguished entities with have nothing to do with the SML-built-in

   188   notion of \qt{bool}.  This results in less readable code;

   189   additionally, eager evaluation may cause programs to loop or break

   190   which would perfectly terminate when the existing SML @{verbatim

   191   "bool"} would be used.  To map the HOL @{typ bool} on SML @{verbatim

   192   "bool"}, we may use \qn{custom serialisations}:

   193 *}

   194

   195 code_type %quotett bool

   196   (SML "bool")

   197 code_const %quotett True and False and "op \<and>"

   198   (SML "true" and "false" and "_ andalso _")

   199

   200 text {*

   201   \noindent The @{command_def code_type} command takes a type constructor

   202   as arguments together with a list of custom serialisations.  Each

   203   custom serialisation starts with a target language identifier

   204   followed by an expression, which during code serialisation is

   205   inserted whenever the type constructor would occur.  For constants,

   206   @{command_def code_const} implements the corresponding mechanism.  Each

   207   @{verbatim "_"}'' in a serialisation expression is treated as a

   208   placeholder for the type constructor's (the constant's) arguments.

   209 *}

   210

   211 text %quote {*

   212   \begin{typewriter}

   213     @{code_stmts in_interval (SML)}

   214   \end{typewriter}

   215 *}

   216

   217 text {*

   218   \noindent This still is not perfect: the parentheses around the

   219   \qt{andalso} expression are superfluous.  Though the serialiser by

   220   no means attempts to imitate the rich Isabelle syntax framework, it

   221   provides some common idioms, notably associative infixes with

   222   precedences which may be used here:

   223 *}

   224

   225 code_const %quotett "op \<and>"

   226   (SML infixl 1 "andalso")

   227

   228 text %quote {*

   229   \begin{typewriter}

   230     @{code_stmts in_interval (SML)}

   231   \end{typewriter}

   232 *}

   233

   234 text {*

   235   \noindent The attentive reader may ask how we assert that no

   236   generated code will accidentally overwrite.  For this reason the

   237   serialiser has an internal table of identifiers which have to be

   238   avoided to be used for new declarations.  Initially, this table

   239   typically contains the keywords of the target language.  It can be

   240   extended manually, thus avoiding accidental overwrites, using the

   241   @{command_def "code_reserved"} command:

   242 *}

   243

   244 code_reserved %quote "\<SML>" bool true false andalso

   245

   246 text {*

   247   \noindent Next, we try to map HOL pairs to SML pairs, using the

   248   infix @{verbatim "*"}'' type constructor and parentheses:

   249 *}

   250 (*<*)

   251 code_type %invisible prod

   252   (SML)

   253 code_const %invisible Pair

   254   (SML)

   255 (*>*)

   256 code_type %quotett prod

   257   (SML infix 2 "*")

   258 code_const %quotett Pair

   259   (SML "!((_),/ (_))")

   260

   261 text {*

   262   \noindent The initial bang @{verbatim "!"}'' tells the serialiser

   263   never to put parentheses around the whole expression (they are

   264   already present), while the parentheses around argument place

   265   holders tell not to put parentheses around the arguments.  The slash

   266   @{verbatim "/"}'' (followed by arbitrary white space) inserts a

   267   space which may be used as a break if necessary during pretty

   268   printing.

   269

   270   These examples give a glimpse what mechanisms custom serialisations

   271   provide; however their usage requires careful thinking in order not

   272   to introduce inconsistencies -- or, in other words: custom

   273   serialisations are completely axiomatic.

   274

   275   A further noteworthy detail is that any special character in a

   276   custom serialisation may be quoted using @{verbatim "'"}''; thus,

   277   in @{verbatim "fn '_ => _"}'' the first @{verbatim "_"}'' is a

   278   proper underscore while the second @{verbatim "_"}'' is a

   279   placeholder.

   280 *}

   281

   282

   283 subsection {* @{text Haskell} serialisation *}

   284

   285 text {*

   286   For convenience, the default @{text HOL} setup for @{text Haskell}

   287   maps the @{class equal} class to its counterpart in @{text Haskell},

   288   giving custom serialisations for the class @{class equal} (by command

   289   @{command_def code_class}) and its operation @{const [source] HOL.equal}

   290 *}

   291

   292 code_class %quotett equal

   293   (Haskell "Eq")

   294

   295 code_const %quotett "HOL.equal"

   296   (Haskell infixl 4 "==")

   297

   298 text {*

   299   \noindent A problem now occurs whenever a type which is an instance

   300   of @{class equal} in @{text HOL} is mapped on a @{text

   301   Haskell}-built-in type which is also an instance of @{text Haskell}

   302   @{text Eq}:

   303 *}

   304

   305 typedecl %quote bar

   306

   307 instantiation %quote bar :: equal

   308 begin

   309

   310 definition %quote "HOL.equal (x\<Colon>bar) y \<longleftrightarrow> x = y"

   311

   312 instance %quote by default (simp add: equal_bar_def)

   313

   314 end %quote (*<*)

   315

   316 (*>*) code_type %quotett bar

   317   (Haskell "Integer")

   318

   319 text {*

   320   \noindent The code generator would produce an additional instance,

   321   which of course is rejected by the @{text Haskell} compiler.  To

   322   suppress this additional instance, use @{command_def "code_instance"}:

   323 *}

   324

   325 code_instance %quotett bar :: equal

   326   (Haskell -)

   327

   328

   329 subsection {* Enhancing the target language context \label{sec:include} *}

   330

   331 text {*

   332   In rare cases it is necessary to \emph{enrich} the context of a

   333   target language; this is accomplished using the @{command_def

   334   "code_include"} command:

   335 *}

   336

   337 code_include %quotett Haskell "Errno"

   338 {*errno i = error ("Error number: " ++ show i)*}

   339

   340 code_reserved %quotett Haskell Errno

   341

   342 text {*

   343   \noindent Such named @{text include}s are then prepended to every

   344   generated code.  Inspect such code in order to find out how

   345   @{command "code_include"} behaves with respect to a particular

   346   target language.

   347 *}

   348

   349 end