[Haskell-cafe] how do I avoid excessive constructor application?
Stefan Holdermans
sholderm at students.cs.uu.nl
Wed Mar 2 09:36:15 EST 2005
Alex,
> fun::(a->a1)->(Either a b)->Either a1 b
> fun f (Left x) = Left (f x)
> fun _ r@(Right x)= Right x
>
> I'd like to avoid the destruction and construction in the third line
> by replacing the right hand side with r. However, the typechecker
> then claims my type is wrong. How do I fix that?
You can't. At the left-hand side r has type Either a b; at the
right-hand side an expression of type Either a1 b is required, so you
can't just supply b.
You could do something like
f :: (a -> c) -> Either a b -> Either c b
f g (Left a) = Left (g a)
f g x = coerceRight x
coerceRight :: Either a b -> Either c b
coerceRight (Right b) = Right b
but I'm not too sure whether that's more elegant.
HTH,
Stefan
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