Re: [Haskell-cafe] Alex -g

On Sun, Feb 20, 2011 at 10:20 PM, Max Bolingbroke wrote:

On 20 February 2011 19:56, Mihai Maruseac wrote:

...

Hi,

When running Alex -g I get several warning telling me that a bang pattern is required and that the warning will be an error in GHC 6.14.

As it happens, that is not an error in GHC 7 (see http://hackage.haskell.org/trac/ghc/ticket/3278):

""" $ ghci -XMagicHash GHCi, version 7.0.1.20101215: http://www.haskell.org/ghc/  :? for help letLoading package ghc-prim ... linking ...  done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... xdone. Loading package ffi-1.0 ... linking ... done. Prelude> let x = 2# in 10

<interactive>:1:5:    Warning: Bindings containing unlifted types should use an outermost bang pattern:               x = 2#    In the expression: let x = 2# in 10    In an equation for `it': it = let x = 2# in 10

<interactive>:1:5:    Warning: Bindings containing unlifted types should use an outermost bang pattern:               x = 2#    In the expression: let x = 2# in 10    In an equation for `it': it = let x = 2# in 10 10 """

(I'm not quite sure why I get two identical warnings..)

It looks like Alex has gone back and forth on what to do here. Selected commits:

""" Fri Nov 26 10:43:42 GMT 2010  Simon Marlow  * don't use CPP for LANGUAGE pragmas  Drop the -fno-warn-lazy-unlifted-bindings again.  CPP in the header  prevents the user from adding their own LANGUAGE pragmas at the top of  the .x file if they're using GHC < 7.0.

Wed Nov 17 10:42:23 GMT 2010  Simon Marlow  * Stop using -fglasgow-exts, and turn off bogus bang-pattern warnings

Wed Nov 17 10:41:06 GMT 2010  Simon Marlow  * remove bang patterns on unlifted bindings again

Wed Jun  3 23:58:54 BST 2009  Ian Lynagh  * Use bang patterns on unlifted bindings """

So it looks like you should just ignore this warning, or turn it off :-)

Thanks :)