wiki:LanguagePragma

Version 5 (modified by ravi@…, 5 years ago) (diff)

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Proposal: LanguagePragma

Ticket #128
Dependencies none
Related specific item in NewPragmas

Compiler support

GHC full
nhc98 none
Hugs none
UHC none
JHC none
LHC full

Summary

Document the minimum guarantees provided by the LANGUAGE pragma.

Description

GHC and Cabal (via Language.Haskell.Extension) have been successfully using the LANGUAGE pragma for the following two purposes:

  1. Document the post-Haskell98 language extensions a source file requires.
  2. Locally enable those required language extensions when compiling an extension-using source file.

The goal of this proposal is to specify the minimum guarantees provided LANGUAGE pragma so that it can be used portably for those same purposes across Haskell 2010 implementations.

References

Report Delta

Replace the the following sentence http://www.haskell.org/onlinereport/pragmas.html 11 Compiler Pragmas:

An implementation is not required to respect any pragma, but the pragma should be ignored if an implementation is not prepared to handle it.

with

With the exception of the LANGUAGE pragma, an implementation is not required to respect any pragma. For non-LANGUAGE pragmas, the pragma should be ignored if an implementation is not prepared to handle it.

Add the following subsection

11.3 Language Extensions

The LANGUAGE pragma is a file-header pragma. A file-header pragma must precede the module keyword in a source file. There can be as many file-header pragmas as you please, and they can be preceded or followed by comments. An individual language pragma begins with the keyword LANGUAGE and is followed by a comma-separated list of named language extensions.

For example, to enable the FFI and preprocessing with CPP:

{-# LANGUAGE ForeignFunctionInterface, CPP #-}

If a Haskell implementation does not recognize or support a particular language extension that a source file requests (or cannot support the combination of language extensions requested), any attempt to compile or otherwise use that file with that Haskell implementation must fail with an error.

In the interests of portability, multiple attempts to enable the same, supported language features (e.g. via command-line arguments, implementation-specific extension dependencies or non-standard pragmas) are specifically permitted.

Haskell 2010 implementations are required to support {-# LANGUAGE Haskell2010 -#}. {-# LANGUAGE Haskell2010 -#} implies support for the following named language extensions standardized in Haskell 2010 (and recognized by pre-Haskell 2010 compilers): {-# LANGUAGE PatternGuards -#}, {-# LANGUAGE EmptyDataDecls #-}, {-# LANGUAGE ForeignFunctionInterface #-}, ...