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What does "from any type 'a' to any type 'b'" mean? That sounds like a range of types, but I don't see how that makes sense. Did you just mean that the map conv

• FreeFull 14y ago
  a and b are placeholders (type variables). They can be any type, for example Integer, or a Float. It doesn't matter. [a] is a list of a, [b] is a list of b. Map takes a function that takes an a and returns a b, and applies it to every element of [a] in order to produce the [b]. An example usage is map lessThanThree [1,2,3,4] In this case, map takes a function where a is an integer, and b is a boolean, then takes a list of integers and produces a list of booleans. The result is [True,True,False,False]
  • jjaredsimpson 14y ago
    It's also illuminating to note that haskell curries function application. This allows us to think of map as a function of one argument. map :: (a -> b) -> ([a] -> [b]) so instead of thinking that map takes a function and a list and applies that function to the list, we can think that map takes a function and "lifts" that functions argument and result types to the list type. This thinking leads us directly to Functors and fmap