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:
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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
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