Version 8 (modified by simonmar@…, 6 years ago) (diff) |
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# get rid of unary '-' operator

the unary '-' is a very odd case in the report and causes a lot of confusion.

to fix it we can

- make '-' part of the lexical syntax of numbers. so a '-' followed by digits is interpreted as a negative number
- get rid of all other special treatment of '-'
- people can use 'negate' if they want to negate non literals.
- the removal of n + k patterns will make this more attractive

Cons

- This does introduce a syntactic oddity, namely that
`-1`is different from`- 1`though syntax highlighting editors could make this distinction obvious.

- It would no longer be possible to write
`x-1`to mean "x minus 1", instead it would mean "x applied to the literal -1", which is counter-intuitive. Also,`x - 1`with spaces would have the "x minus 1" meaning.

- In Haskell 98, '-' is explicitly excluded from right sections, so:

Prelude> :t (- 1) (- 1) :: (Num a) => a Prelude> :t (1 -) (1 -) :: (Num a) => a -> a

If we removed special treatment of '-', then these become symmetrical again. However it seems unintuitive that

(-x)is a right section of '-', but(-1)is the literal -1.

Another motivation for lexing - as part of a numeric literal (or changing its precedence), from 2007-10-10 #haskell:

[09:33] < quicksilver> am I the only person who finds mod's behaviour with negative numbers stupid [09:34] < matthew-_> try rem then [09:34] < quicksilver> > (-4) `mod` 16 [09:34] < quicksilver> > (-4) `rem` 16 [09:34] < lambdabot> 12 [09:34] < lambdabot> -4 [09:34] < quicksilver> that's odd, that's not what my ghci did [09:34] < quicksilver> doh [09:34] < quicksilver> bracketing error :P [09:35] < quicksilver> mod does do what I expect, if I put the negative number in bracets [09:35] < quicksilver> I wouldn't expect 'unary minus' to have lower precedence that `mod` though [09:35] < sieni> > -4 `rem` 16 [09:35] < lambdabot> -4 [09:35] < sieni> oops [09:35] < sieni> > -4 `mod` 16 [09:35] < lambdabot> -4 [09:35] < quicksilver> sieni: yeah, that's what I did [09:36] < sieni> quicksilver: yeah I know, that has bitten me too :-) [09:36] < quicksilver> I always expect unary minus to bind tight [09:37] < osfameron> yeah, haskell's unary minus is surprisingly loose

haskell-src-exts, haskell-src, and HLint have been using a unary minus that binds tighter than any other infix operator, and have never encountered any problems with it: http://www.haskell.org/pipermail/haskell-prime/2009-August/002955.html.