fixity resolution

[Simon Marlow]

On 07/07/2010 10:09, Christian Maeder wrote:
> Christian Maeder schrieb:
>> http://www.haskell.org/~simonmar/haskell-2010-draft-report-2/haskellch10.html#x17-17300010
>>
>> "Fixity resolution also applies to Haskell patterns, but patterns are a
>> subset of expressions so in what follows we consider only expressions
>> for simplicity."
>
> I suggest to change "also applies to Haskell patterns" to
>
> "also applies to Haskell patterns and left hand sides of infix function
> bindings"
>
>> The string "1 * - 1" is legal as pattern, but rejected as expression!
>
> Further points:
>
> 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?

> 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.

If you make a proposal to change this, then I would probably support it.

Cheers,
	Simon


> An infix-expression following an unary minus is resolved independently
> first. If the top-level operator has a strictly higher precedence than
> "-" its resolved as minus term, otherwise the same procedure is applied
> to the left argument of the infix expression. (If the left argument is
> no infix expression, we are done by plain prefix application of minus.)
>
>>
>> Furthermore fixity resolution does not distinguish between constructors
>> and other operators as it should according to the grammar:
>>
>> pat 	 → 	lpat qconop pat     	 (infix constructor)
>> 	| 	lpat
>>
>>
>> funlhs 	 → 	var apat { apat }
>> 	| 	pat varop pat
>
> add a description:
> 	|	pat varop pat		(infix binding)
>
>> 	| 	( funlhs ) apat { apat }
>>
>>
>> "a : b * c : d = undefined" is currently rejected with:
>
> A similar example is given in 4.4.3.1  Function bindings. It should be
> referenced in 10.6  Fixity Resolution
>
> Cheers Christian
>
>>
>> "cannot mix `:' [infixr 5] and `Main.*' [infixl 9] in the same infix
>> expression"
>>
>> but should be fine by the given grammar (rule "pat varop pat").
>>
>> Cheers Christian
>>
>> P.S. like in my proposal for infixexp I would change pat to:
>>
>> pat 	 → 	pat qconop pat     	 (infix constructor)
>> 	| 	lpat
>>
>> for the sake of a better presentation only.
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