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5. Green Card directives

Green Card pays attention only to Green Card directives, each of which starts with a ``%'' at the beginning of a line. All other lines are passed through to the output Haskell file unchanged.

The syntax of Green Card directives is as follows (the syntax for dis is given in Section 9.1):

Program  :  Declaration_1
            Declaration_n     n >= 1

Declaration : Procedure
            | '%const' [Var] [Var1 .. Varn]
            | '%enum'  [Var] Derivings
                      Type  [Var1 .. Varn]
            | '%dis' [Var1] [Var1 .. Varn] '=' dis
            | '%#include' Filename
            | '%prefix' Var
            | '%C' ccode

Procedure   : [Signature] [call] [ccode] [result]

Signature   : '%fun' Var :: Type

Type        : Var
            | Var Type
            | Type '->' Type
            | '(' Type_1, ... , Type_n ')'    n >= 1

Call        : '%call' Dis_1 ... Dis_n

Result      : '%fail' Cexp Cexp [Result]
            | '%end' Cexp
            | '%result' Dis

Cexp        : anything up until '}' is encountered.
            | Ccode

Ccode       : '%code' Var
            | '%safecode' Var

Filename    : '<' Var '>'
            | '"' Var '"'

Green Card understands the following directives:

  • %fun begins a procedure specification, which describes the interface to a single C procedure, see Section 6.

  • %dis allows the programmer to describe a new Data Interface Scheme (DIS). A DIS describes how to translate, or marshall, data from Haskell to C and back again, see Section 9.

  • %const makes it easy to generate a collection of new Haskell constants derived from C constants. This can be done with %fun, but %const is much more concise, see Section 8.1.

  • %enum is similar to %const, but supports the mapping of external constants to a corresponding Haskell data type, see Section 8.2.

  • %prefix makes it easy to remove standard prefixes from the Haskell function name, those are usually not needed since Haskell allows qualified imports, see Section 8.3.

  • Procedure specifications can, as we shall see, contain fragments of C. %#include tells Green Card to arrange that a specified C header file will be included with the C code in the procedure specifications when the latter is fed to a C compiler, see Section 11.

  • ``%C'' allows you to write fragments of C code which sits outside any procedure specification. (We shall later see how to include fragments of C code within procedures.) The entire line of text following this directive is simply copied verbatim to the generated C module.

A directive can span more than one line, but the continuation lines must each start with a % followed by some whitespace. For example:

%fun draw :: Int                -- Length in pixels
%           -> Maybe Int        -- Width in pixels
%           -> IO ()

Haskell-style comments are permitted in Green Card directives (except, for obvious reasons, ``%C''.)

A general principle we have followed is to define a single, explicit (and hence long-winded) general mechanism, that should deal with just about anything, and then define convenient abbreviations that save the programmer from writing out the general mechanism in many common cases. We have erred on the conservative side in defining such abbreviations; that is, we have only defined an abbreviation where doing without it seemed unreasonably long-winded, and where there seemed to be a systematic way of defining an abbreviation.