[Haskell-cafe] Fixity declaration extension

[Евгений Пермяков]

Your idea looks _much_ better from code clarity point of view, but it's 
unclear to me, how to deal with it internally and in error messages. I'm 
not a compiler guy, though.

Worse, it does not allow to set up fixity relative to operator that is 
not in scope and it will create unnecessary intermodule dependencies.  
One should fall back to numeric fixities for such cases, if it is needed.

On 08/13/2012 11:26 PM, Ryan Ingram wrote:
> When I was implementing a toy functional languages compiler I did away 
> with precedence declarations by number and instead allowed the 
> programmer to specify a partial order on declarations; this seems to 
> be a much cleaner solution and avoids arbitrary precedences between 
> otherwise unrelated operators defined in different modules.
>
> You could write statements like
>
> -- define + and - to have the same precedence
> infixl + -
>
> -- define * to have higher precedence than +
> infixl * above +
>
> -- define / to have the same precedence as *
> infixr / equal *
>
> -- $ is right-associative
> infixr $
> -- you can also separate precedence from fixity declaration
> precedence $ below +
>
> -- function application has higher precedence than all operators by 
> default, but you can override that
> infixl . above APP
>
> -- == is non-associative
> infix ==
>
> Here's some parses with this system:
>
> a + b - c   =>   (a+b)-c
> f.x.y z == g w  => (((f.x).y) z) == (g w)
> a == b == c  => parse error (non-associative operator)
> a * b / c => parse error (left-associative/right-associative operators 
> with same precedence)
> a == b $ c => parse error (no ordering known between == and $)
> a $ b + c => a $ (b+c)
>
> I think this is a much cleaner way to solve the problem and I hope 
> something like it makes it into a future version of Haskell.
>
>   -- ryan
>
> On Sun, Aug 12, 2012 at 11:46 AM, Евгений Пермяков 
> <permeakra at gmail.com <mailto:permeakra at gmail.com>> wrote:
>
>     fixity declaration has form *infix(l|r)? [Digit]* in haskell. I'm
>     pretty sure, that this is not enough for complicated cases.
>     Ideally, fixity declarations should have form *infix(l|r)?
>     [Digit](\.(+|-)[Digit])** , with implied infinitely long repeated
>     (.0) tail. This will allow fine tuning of operator priorities and
>     much easier priority selection. For example, it may be assumed,
>     that bit operations like (.&.) operator have hightest priority and
>     have priorities like 9.0.1 or 9.0.2, anti-lisps like ($) have
>     lowest priority like 0.0.1, control operators have base priority
>     1.* and logic operations like (&&) have priority of 2.* and it
>     will be possibly to add new operators between or above all (for
>     example) control operators without moving fixity of other ones.
>
>     Agda2 language supports wide priority range, but still without
>     'tails' to my knowledge. Is there any haskell-influenced language
>     or experimental syntactic extension that address the issue?
>
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>

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