Difference between list.map and list.collect

Question

What is the difference between map and collect? documentation below - i cant tell the difference..

---

List.map<'T,'U> Function

Creates a new collection whose elements are the results of applying the given function to each of the elements of the collection.

// Signature: List.map : ('T -> 'U) -> 'T list -> 'U list

---

List.collect<'T,'U> Function

For each element of the list, applies the given function. Concatenates all the results and returns the combined list.

Answer 1

The difference is that the output list from map is the same length as the input list. For collect, the output list may be longer or shorter than the input as the function you pass returns a list instead of a single element.

For more detail - compare the signatures. For map

List.map : ('T -> 'U) -> 'T list -> 'U list

and collect

List.collect : ('T -> 'U list) -> 'T list -> 'U list

you can see here that the return type of the function argument is different.

Answer 2

An example for illustration

Let's say you have a list of lists:

let list = [ [1; 2]; [3; 4]; [5; 6] ]

And a transformation function:

let trans = fun x -> [for i in x -> i*10]

---

List.map trans list will produce:

[[10; 20]; [30; 40]; [50; 60]]

While

List.collect trans list will produce:

[10; 20; 30; 40; 50; 60]

---

List.collect will apply the transformation function and will Concatenate the result while List.map will only apply the transformation function.

Answer 3

Adding to @John's answer, the difference is in the word Concatenates.

Basically, List.collect f xs is the same as List.concat (List.map f xs).