FWIW, this can be inferred fully automatically. Type let pair = fun x -> fun y -> { _0 = x; _1 = y } let rec reduce = fun init -> fun combine -> fun x

FWIW, this can be inferred fully automatically. Type let pair = fun x -> fun y -> { _0 = x; _1 = y } let rec reduce = fun init -> fun combine -> fun xs -> match xs with [] -> init | x :: rest -> reduce (combine x init) combine rest let test = reduce 0 pair [1; 2; 3; 4] into https://www.cl.cam.ac.uk/~sd601/mlsub/ https://www.cl.cam.ac.uk/~sd601/mlsub/ to see a demonstration.