Question

Ok, I can’t figure this one out even though I have an idea what it’s doing…

let t = ["APE", "MONKEY", "DONKEY"]

Now consider three cases:

map (length.group) t
(map length.group) t
map (map length.group) t

This returns these three answers:

[3,6,6]
[1,1,1]
[[1,1,1],[1,1,1,1,1,1],[1,1,1,1,1,1]]

Now, can someone explain to me in details what’s going on. A crucial part of this question is that I assume that map needs a list to work on and I don’t see two maps being passed in the third case for example.

Answer 1

map (length.group) t

This composes the functions length and group. The result is a function that takes a list (string) and returns the number of “groups” in that list (where a group is a sequence of the same character repeating 1 or more times, so “abc” contains 3 groups and so does “aabbcc”).

This function is then applied to each string in t using map.

(map length.group) t

Here the function map length (which takes the length of each sublist in a list of lists) is composed with the function group and the composed function is applied to t. In other words it’s the same as map length (group t).

map (map length.group) t

Here the function map length . group is applied to each string in t, i.e. map length (group str) is calculated for each string str in t.