The previous blogpost implements a hylomorphism in Erlang.

Following is again the same in F#: but this is part of my exercices to learn F#, and I just begin to learn … so the code is … ergh, whatever.

And I have no idea how to implement default values. Any advice?

#light
open Microsoft.FSharp.Collections.List;
let rec hylo step till col inj v s =
if till s
then
v
else
let ns = step s in
let nv = inj v (col s) in
hylo step till col inj nv ns;
let fact =
let step = fun x -> x - 1 in
let till = fun x -> x <= 1 in
let col = fun x -> x in
let inj = fun a x -> a * x in
hylo step till col inj 1;
let evens n =
let step = fun x -> x + 2 in
let till = fun x -> x >= n in
let col = fun x -> x in
let inj = fun a x -> x :: a in
rev (hylo step till col inj [] 0);
let to_bin n =
let step = fun x -> x / 2 in
let till = fun x -> x <= 0 in
let col = fun x -> x % 2 in
let inj = fun a x -> x :: a in
hylo step till col inj [] n;
let expand l =
let rec duplicate c n a =
if n > 0
then duplicate c (n - 1) (c::a)
else a
in
let step = fun li -> tl li in
let till = fun li -> match li with |[] -> true |_ -> false in
let col = fun li -> match hd(li) with |(c,n) -> duplicate c n [] in
let inj = fun a x -> (x :: a) in
rev (hylo step till col inj [] l);
do printf "fact(5)=%a\n" output_any (fact 5);
do printf "evens(10)=%a\n" output_any (evens 10);
do printf "to_bin(10)=%a\n" output_any (to_bin 10);
do printf "expand([(1,2); (4,7)])=%a\n" output_any (expand [(1, 2); (4,7)]);

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