[Haskell-cafe] Re: Fractional/negative fixity?
westondan at imageworks.com
Tue Nov 7 19:20:52 EST 2006
I started this e-mail thread on HaskellCafe instead of HaskellPrime
because it was minimal, backwards-compatible, valid Haskell 98 (or very
nearly so) and could go (now) into GHC if someone saw fit to put it in.
> If you think C++ is not overly complicated, just what is a protected
> abstract virtual base pure virtual private destructor, and when was
> the last time you needed one?
> -- Tom Cargil, C++ Journal
The above was not a proposal for Haskell'.
> Proposers of new [C++] features should be required to donate a
kidney. > That would - Jim [Waldo] pointed out - make people think hard
> proposing, and even people without any sense would propose at most two
> -- Bjarne Stroustrup, creator of C++
But at the risk of losing a kidney...
Here's where I think Haskell 98 got it wrong:
"A fixity declaration may appear anywhere that a type signature appears
and, like a type signature, declares a property of a particular operator."
Like a type signature? A type signature has semantic value, fixity does
not. In principle, neither does the name that a lambda abstraction is
bound to in top-level definitions, but we are stuck with linkers that
know about names. They don't know about fixity.
Operator precedence is a purely syntactic device for people (like me)
who hate parentheses. Its "scope" is essentially local to the attention
span of a human being (i.e. a single module). A preprocessor can
parethesize code to eliminate the need for fixity. I hate chasing other
modules looking up operator fixity and would rather just specify it myself:
What if we could remap fixity on import:
-- (MyModule.???) fixity is 4, exported as its default...
-- But in this module I prefer 5...
-- This fixity is *not* exported!
infix 5 ???
Modules that import this module get the default fixity from the source.
Summary: fixity at definition is exported, fixity on import is locally
remapped but not reexported.
> Better is the enemy of the good
> -- Voltaire
Simple, powerful, minimal. Any takers?
Dan Weston wrote:
> Here's an idea that (I think) is useful and backwards compatible:
> fractional and negative fixity.
> There have been 3 separate times where I've wanted an operator just
> above 0 ($) but less than 1 (>>= or >>>), or else just below 0 (like a
> superlow $$)
> infix 0.5 ???
> infix -1 $$
> The only change would be internal to compiler, wouldn't it? Since fixity
> is just syntactic sugar, there should be no semantic difficulties.
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