Re: Re[2]: [Haskell-cafe] Lambda-case / lambda-if

At 12:05 PM +0200 10/4/10, Christopher Done wrote:

On 4 October 2010 10:55, Bulat Ziganshin wrote:

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Hello Ketil,

Monday, October 4, 2010, 11:30:48 AM, you wrote:

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Prelude> (if then "Haskell" else "Cafe") False

lambda-if is easily implemented in terms of usual functions. and we even have one named bool:

bool: Bool -> a -> a -> a

I agree, in fact I have bool here: http://hackage.haskell.org/packages/archive/higherorder/0.0/doc/html/Data-Bo...

And the corresponding other types:

bool :: (a -> b) -> (a -> b) -> (a -> Bool) -> a -> b list :: b -> ([a] -> b) -> [a] -> b maybe :: b -> (a -> b) -> Maybe a -> b

But the case is especially useful for pattern matching.

I agree with others that lambda-if is better provided as a normal function rather than special syntax, that lambda-case is much more useful, and that it would be best if lambda-case were simply a generalization of anonymous lambdas (\ ...). With respect to "datatype destructing" functions, the Prelude has: maybe :: b -> (a -> b) -> Maybe a -> b either :: (a -> c) -> (b -> c) -> Either a b -> c which suggests the following signatures for the analogues for Bool and list types: bool :: a -> a -> Bool -> a list :: b -> (a -> [a] -> b) -> [a] -> b (However, I do rather like the name (??) from Data.Bool.Higher, which is used for "bool" immediately above. And I would hesitate to use the name "list" for list destruction rather than construction. So I'm not about to propose adding these two functions.) Dean