Functors

The prelude defines:

class Functor f
  map : (a -> b) -> f a -> f b

map applies a pure function inside a container f.

If you can describe an operation as “change the values without changing the structure”, it’s a good fit for map.

Mapping over lists

map ((*) 2) [1, 2, 3]

Mental model

Each element is transformed independently; list length stays the same.

Mapping over Option

( map ((+) 1) (Some 41)
, map ((+) 1) None
)

None acts like “no value to transform”.

Mapping over Result

( map ((*) 2) (Ok 21)
, map ((*) 2) (Err "boom")
)

map transforms Ok values but leaves Err unchanged.

A useful pattern: composing transforms

Instead of branching on shapes early, keep your code “in the functor”:

let inc = \x -> x + 1 in
  map inc (Some 1)