Non-H98 crusade, contd.
Keean Schupke
k.schupke at imperial.ac.uk
Mon Feb 28 07:40:42 EST 2005
Ross Paterson wrote:
>Where can I read this understanding? The GHC docs on FDs are terse,
>referring to the original paper, but that paper is somewhat informal,
>and describes a weaker system than what is implemented. For example,
>(taken from s7 of the paper), given
>
> class U a b | a -> b where u :: a -> b
> class U a b => V a b where v :: a -> b
>
>the paper says that the principal type of \x -> (u x, v x) is
>
> (U a b, V a c) => a -> (b,c)
>
>According to GHC and Hugs, it is
>
> V a b => a -> (b,b)
>
>I think that's sensible, but where are the rules that give it?
>I suspect that writing this addendum may take a while.
>
>
Thats just a type simplification, in the source, we have
\x -> (u x, v x)
the type checker knows both x's are the same, and it knows
that (V a b) is the same as (U a b)...
So in this case it looks like the second type is just a simplification
of the first... so they really agree on the type, not disagree.
Keean.
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