Functor

[distums]

definition from fantasy-land

1. u.map(a => a) is equivalent to u (identity)
2. u.map(x => f(g(x))) is equivalent to u.map(g).map(f) (composition)

map method

map :: Functor f => f a ~> (a -> b) -> f b

A value which has a Functor must provide a map method. The map method takes one argument:

u.map(f)

1. f must be a function,

• If f is not a function, the behaviour of map is unspecified.
• f can return any value.
• No parts of f's return value should be checked.
  2. map must return a value of the same Functor

[distums]

Functor是一个容器，对于Functor，非常重要的一点是弄清它所包装的值