Wanted: warning option for usages of unary minus

[Simon Marlow]

I definitely think that -1# should be parsed as a single lexeme.  Presumably it 
was easier at the time to do it the way it is, I don't remember exactly.

I'd support a warning for use of prefix negation, or alternatively you could 
implement the Haskell' proposal to remove prefix negation completely - treat the 
unary minus as part of a numeric literal in the lexer only.  This would have to 
be optional for now, so that we can continue to support Haskell 98 of course.

Cheers,
	Simon

Isaac Dupree wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> Now I understand why negative unboxed numeric literals are parsed
> weirdly, after poking around a little!
> "The parser parses all infix applications as right-associative,
> regardless of fixity."
> <http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/Renamer>
> 
> A negative sign on the left of an expression is parsed as a special
> case, binding tighter than all infix ops (until the renamer reassociates
> it) (but '-' is not parsed that way when it _follows_ an expression: (
> process -1# ) is treated as _infix_ minus, i.e. subtraction, i.e. likely
> compile error).
> 
> Then, before reassociating based on fixity, negation of an unboxed
> number is performed (in order to allow a sort of numeric literals that
> are negative and unboxed).  Here is a result of this funny treatment:
> 
> \begin{code}
> {-# OPTIONS_GHC -fglasgow-exts #-}
> 
> import GHC.Base
> 
> main = do
>   putStrLn $ "boxed:   " ++ show (    ( - 2  ^  6  ) :: Int )
>   -- output:  boxed:   -64   --  ===  ( -(2  ^  6 ))
> 
>   putStrLn $ "unboxed: " ++ show ( I# ( - 2# ^# 6# ) )
>   -- output:  unboxed: 64    --  ===  ((- 2#)^# 6# )
> 
> 
> infixr 8  ^#  --just like ^, binds tighter than - (which is infixl 6)
> ( ^# ) :: Int# -> Int# -> Int#
> base ^# 0# = 1#
> base ^# exponent = base *# (base ^# ( exponent -# 1# ))
> \end{code}
> 
> This particular combination of behavior, unfortunately, doesn't seem
> useful for implementing sensible numeric literals, IMHO.  My desired
> warning scheme would have to wait for the renamer to sort out
> fixities... unless I want to warn about (-1==1) which is ((-1)==1), as
> well (do I want that warning? how about (1 == -1), or (1 ^^ -1), which
> both must parse with negation being tightly binding? I hadn't considered
> those very well yet...).
> 
> 
> Isaac
> -----BEGIN PGP SIGNATURE-----
> Version: GnuPG v1.4.3 (GNU/Linux)
> Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org
> 
> iD8DBQFGGOF8HgcxvIWYTTURAiT5AKC1Zl9JYuSLBPdey/YdmCriY7FaUQCgqzNQ
> clHWTS162IZWHhlXKJR8NhQ=
> =zqzy
> -----END PGP SIGNATURE-----
> _______________________________________________
> Glasgow-haskell-users mailing list
> Glasgow-haskell-users at haskell.org
> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
>