Marco Tu'lio Gontijo e Silva wrote:
is there a way to defined something as a map to use in tuples?
Yes, it is: and it is quite easy and straightforward. Udo Stenzel
since c would be a variable that ranges over type classes, and that doesn't exist.
Of course it does: please see below (as well as several earlier posts on the Haskell list showing that Haskell classes *are* first-class). Here's the definition of mapTuple
mapTuple l (x,y) = (apply l x, apply l y)
and here its inferred type:
*MapTuple> :t mapTuple mapTuple :: (Apply l b y1, Apply l a y) => l -> (a, b) -> (y, y1)
Our mapTuple applies to (in general) heterogeneous tuples and gives (in general) a heterogeneous tuple. Here's the complete code and the tests. {-# OPTIONS -fglasgow-exts #-} module MapTuple where class Apply l x y | l x -> y where apply:: l -> x -> y data SHow = SHow instance Apply SHow String String where apply _ = show instance Apply SHow Int String where apply _ = show instance Apply SHow Bool String where apply _ = show mapTuple l (x,y) = (apply l x, apply l y) -- Here, the result is a homogeneous tuple test1 = mapTuple SHow ("a",(1::Int)) test2 = mapTuple SHow ((1::Int),"b") test3 = mapTuple SHow (True,"b") -- ANother operation: SUcc data SUcc = SUcc instance Apply SUcc String String where apply _ x = x ++ x instance Apply SUcc Int Int where apply _ = succ instance Apply SUcc Bool Bool where apply _ = not -- Here, the result is a heterogeneous tuple (as is the argument) testH1 = mapTuple SUcc ("a",(1::Int)) testH2 = mapTuple SUcc ((1::Int),"b") testH3 = mapTuple SUcc (True,"b")