[Haskell-cafe] Lambda-case / lambda-if

[Dean Herington]

At 12:05 PM +0200 10/4/10, Christopher Done wrote:
>On 4 October 2010 10:55, Bulat Ziganshin <bulat.ziganshin at gmail.com> wrote:
>>  Hello Ketil,
>>
>>  Monday, October 4, 2010, 11:30:48 AM, you wrote:
>>>>  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-Bool-Higher.html
>
>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