Lets say we have a record which defines a person type with the properties name and age:
type person = {name:string ; age:int };;
and initialize a list with different types:
let personlist = [{name="alexander";age=21};{name="benjamin";age=30};{name="claudia";age=21}];;
How can I count the amount of types with a specific age, lets say 21 (alexander and claudia) so that the output in this case would be two?
The most straightforward way would be to combine List.length and List.filter.
List.(personlist |> filter (fun p -> p.age = 21) |> length)
However, this is somewhat less efficient than it could be. We can do it in one pass because we don't really need to build a list of the matching people, and then iterate over that list to find its length.
List.fold_left will let us iterate over personlist, updating the initial value of 0 on each iteration dependent on whether the age field of each record is 21 or not.
List.fold_left (fun i {age; _} -> i + if age = 21 then 1 else 0) 0 personlist
This can be generalized as a count function which takes a predicate function as an argument.
let count p lst =
lst
|> List.fold_left (fun i x -> i + Bool.to_int (p x)) 0