| > My claim was that
| >
| > forall a. Show a => T
| >
| > could be implemented by passing a bottom dictionary for Show.
|
| Excuse me, but Jan-Willem Maessen has already shown that this
| implementation can give unexpected results.
Yes, I was quite wrong about that. How embarassing.
But there's still something lurking there. Consider:
data T a = T1 Int | T2 a
It's clear that (T1 Int) has no a's in it, not even bottom. Mark
Shields
and ruminated in the corridor about a kind system to make this apparent.
That is, T1 would have type
T1 :: forall a::Pure . Int -> T a
Then if we see (forall a::Pure. Show a => <type>)
we're justified in fixing a to Empty. You need a sub-kinding system to
make
this work, so the cost has just gone up. My implemention mood has
suddenly past.
Isn't laziness wonderful?
Simon
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