Safe Haskell

[Simon Peyton-Jones]

Simon's message yesterday boils down to
	"solve the problem using CPP"
Although hardly beautiful, it seems to work and means we don't need another new feature.  Is that acceptable to David & David?

Simon

| -----Original Message-----
| From: cvs-ghc-bounces at haskell.org [mailto:cvs-ghc-bounces at haskell.org] On Behalf Of
| Simon Peyton-Jones
| Sent: 26 July 2011 08:13
| To: David Terei; Ian Lynagh
| Cc: cvs-ghc at haskell.org; David Mazieres expires 2011-10-22 PDT
| Subject: RE: Safe Haskell
| 
| [Widening to cvs-ghc because others may have opinions]
| 
| | > Generally, if you want to put a language extension in a pragma then the
| | > compiler needs to support that extension, or compilation will fail.
| | > There may be some odd exceptions (the main one that comes to mind is
| | > {-# LANGUAGE NoSomeExtensionYouDoNotHave #-}; most of the Safe and
| | > Trustworthy functionality is similar to that, really) but I'm not
| | > convinced that LANGUAGE_MAYBE is the right solution.
| |
| | I understand the concern here but would like to hear from the Simon's
| | before weighing in. Simon's if you could offer you opinions that would
| | be great as I'd like to carry on with the work on making the libraries
| | included with GHC safe.
| 
| Simon M is on holiday this week, and I have not been following the discussion
| properly.  I believe the question at issue is this.
| 
| * "LANGUAGE Foo" means that the module uses language extension Foo, and won't
|   compile unless the compiler supports Foo
| 
| * But Safe Haskell for the first time is a language *restriction*.  You are
|   questioning {-# LANGUAGE SafeHaskell #-}  (or trustworthy or something)
|   because it *reduces* the set of programs accepted, but does not *increase* them.
|   So an old compiler that knows nothing of SafeHaskell would compile the module
|   just fine.
| 
| * So you don't want to add lots of {-# LANGUAGE SafeHaskell #-} pragmas that would
| mean
|   you couldn't compile the module with an older compiler.
| 
| * Ian responded that if you use "import safe" then you are *increasing* the
|   language accepted.  You didn't respond, except to say that you "don't have to use
|   import safe".  But that is a problem isn't it?
| 
| It occurs to me that restricting the language is a bit like "-fwarn-unused-bindings -
| Werror".  That also restricts the language, this time to programs that don't have any
| unused bindings.  So another alternative would be
| 
| 	{-# OPTIONS_GHC -safe #-}
| 
| But again, older GHCs will simply reject such programs with "unknown flag".  But
| perhaps un-recognised flags should only be warned about, rather than errored about?
| After all, we could imagine adding lots of cunning flags that do extra checks, but
| older compilers should still compile the module.
| 
| So your proposal is
|   {-# LANGUAGE_MAYBE SafeHaskell #-}
| where LANGUAGE_MAYBE flags are ignored (albeit with a warning) if unrecognised.  I
| kind of like that.  Maybe LANGUAGE_RESTRICTION?
| 
| I'm a bit reluctant to commit to something before Simon M gets back (in a week's
| time).
| 
| Simon
| 
| 
| 
| 
| 
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