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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. Ma
by 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 agoIt'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