Simon Marlow schrieb: [...]
1. "- 1 * 1" is accepted as legal pattern, but differently resolved for expressions! Should one not reject these (rare) patterns, too?
That's the GHC bug, right?
Yes!
2. I would rather allow "1 * - 1" and "1 + - 1" to be legal as expressions (with its unambiguous interpretation).
Yes, me too, but that's a matter for a new proposal.
3. Associativity should not matter for the non-binary "-"!
So the following resolutions are possible:
"1 + - 2 + 3" ~~~> "(1 + -2) + 3" "1 + - 2 * 3" ~~~> "1 + -(2 * 3)"
infix 6 ## -- same precedence like "+" but different associativity
"- 1 ## 2" ~~~> "(-1) ## 2"
Yes, again I agree. The current fixity resolution is more strict than it needs to be. The intention in Haskell 2010 was not to change the way fixity resolution worked, but rather to avoid the problems caused by having it as part of the grammar.
The grammar (in particular an ambiguous one) describes a superset of the language and need not change with a changed fixity resolution (or type analysis).
If you make a proposal to change this, then I would probably support it.
A larger case would be "1 * - 2 * 3", that I would resolve to "1 * - (2 * 3)" by resolving everything after "-" first. This is sort of an arbitrary choice, but probably ok and in the same spirit than resolving "- 2 * 3" to "- (2 * 3)". C.