Say you've got some moderately-complex function for searching through a list, for example:
let find_nth (test : 'a -> bool) (n : int) (lst : 'a list) : (int * 'a) ->
- let rec helper (position : int) n lst =
- match lst with
- | [] -> failwith "not found"
- | x :: xs when test x -> (if n = 1
- then (position, x)
- else helper (position + 1) (n - 1) xs
- )
- | x :: xs -> helper (position + 1) n xs
- in helper 0 n lst;;
+ let rec helper (position : int) n lst =
+ match lst with
+ | [] -> failwith "not found"
+ | x :: xs when test x -> (if n = 1
+ then (position, x)
+ else helper (position + 1) (n - 1) xs
+ )
+ | x :: xs -> helper (position + 1) n xs
+ in helper 0 n lst;;
This searches for the `n`th element of a list that satisfies the predicate `test`, and returns a pair containing the position of that element, and the element itself. Good. But now what if you wanted to retrieve a different kind of information, such as the `n`th element matching `test`, together with its preceding and succeeding elements? In a real situation, you'd want to develop some good strategy for reporting when the target element doesn't have a predecessor and successor; but we'll just simplify here and report them as having some default value:
let find_nth' (test : 'a -> bool) (n : int) (lst : 'a list) (default : 'a) : ('a * 'a * 'a) ->
- let rec helper (predecessor : 'a) n lst =
- match lst with
- | [] -> failwith "not found"
- | x :: xs when test x -> (if n = 1
- then (predecessor, x, match xs with [] -> default | y::ys -> y)
- else helper x (n - 1) xs
- )
- | x :: xs -> helper x n xs
- in helper default n lst;;
+ let rec helper (predecessor : 'a) n lst =
+ match lst with
+ | [] -> failwith "not found"
+ | x :: xs when test x -> (if n = 1
+ then (predecessor, x, match xs with [] -> default | y::ys -> y)
+ else helper x (n - 1) xs
+ )
+ | x :: xs -> helper x n xs
+ in helper default n lst;;
This duplicates a lot of the structure of `find_nth`; it just has enough different code to retrieve different information when the matching element is found. But now what if you wanted to retrieve yet a different kind of information...?