How to interpret this Ramda signature?

Question

Could someone explain how to understand this notation:

((a, b) → a) → a → [b] → a

See: https://ramdajs.com/docs/#reduce

Answer 1

((a, b) → a) → a → [b] → a
^^^^^^^^^^^^   ^   ^^^   ^
1              2   3     4

This is a function that takes three arguments (1) (2) (3) and returns a value of type a (4):

1. The 1st arg is a function that takes two arguments (maybe of the same type) and returns a value of the same type as the first argument.
2. The 2nd argument is a value of type a
3. The 3rd argument is a list of values of type b

reduce( (acc, x) => acc + x.length,    0,   ["foo", "bar", "baz"]); //=> 9
//       ^^^  ^     ^^^^^^^^^^^^^^     ^    ^^^^^^^^^^^^^^^^^^^^^        ^
//       a    b     a                  a    [b]                          a
//     ((a -> b) -> a             ) -> a -> [b]                       -> a

In this case a stands for the number type and b stands for the string type.