Re: [Haskell-cafe] how do I avoid excessive constructor application?

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