Manuel M T Chakravarty
chak at cse.unsw.edu.au
Mon Dec 3 23:31:19 EST 2007
> Is it really a good idea to permit a type signature to include
> equality constraints among unifiable types? Does the above type
> signature mean something different from a ->a? Does the type
> foo :: (a~Bar b) => a -> Bar b
> mean something different from:
> foo :: Bar b -> Bar b
> ? I know that System FC is designed to let us write stuff like:
> foo :: (Bar a ~ Baz b) => Bar a -> Baz b
> Which is of course what we need for relating type functions. But
> I'm wondering if there's a subtlety of using an equality constraint
> vs just substitution that I've missed---and if not why there are so
> many ways of writing the same type, many of them arguably unreadable!
Simon answered most of your question, but let me make a remark
regarding "why there are so many ways of writing the same type, many
of them arguably unreadable!" Equalities of the form "a ~ someType"
are essentially a form of let-bindings for types - you can give a type
a name and then use the name in place of the type. Just like with
value-level let bindings, you can abuse the notation and write
unreadable terms. However, this is no reason to remove let-bindings
from the value level, so why should it be different at the type level?
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