Theory

A list is an ordered, homogeneous sequence.

[]      -- empty
(:)     -- cons  (x : xs)

[a] is syntactic sugar for [] | (:) a [a].

[1..5]      ==  [1,2,3,4,5]
[2,4..10]   ==  [2,4,6,8,10]

[(i,j) | i <- [1..3], j <- [1..i], gcd i j == 1]

Semantics: generate (<-), filter (guards), yield expression.

Implementation

The core library offers null, head, tail, length, (++).

Idiomatic pattern matching:

len :: [a] -> Int
len []     = 0
len (_:xs) = 1 + len xs