glimming at kth.se
Tue Feb 8 14:06:25 EST 2005
I have a problem with contravariance in the bananas in space setting.
If you take catamorphism from a function space such as A=A->A to a type
such as BxC, then the negative occurrence of X in the functor GXY=X->Y
swaps around BxC into B+C.
This means that maybe in this particular case Meijer-Hutton's original
Haskell code for cata/ana does not work, since the type for cata would
force BxC and B+C to be the same type.
I might be wrong about this, and I wonder anybody can comment on this.
Med vänliga hälsningar / Yours Sincerely,
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