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ppx_ski

SKI combinator calculus implemented in OCaml supported by a ppx extension.

Syntax

The new syntaxes introduced by ppx_ski are:

  • let%ski C = <SKI-expression>
  • {|SKI-expression|}
  • {l|SKI-expression|l} (which returns a lambda)

REPL

There's a simple repl included to let you evaluate SKI-expressions. To build it: run make repl (just a Makefile wrapper around ocamlbuild).

SKI> ((S((SK)K))((SK)K))a
(a(a))
SKI> ((S(K((S((S(K((S(KS))K)))S))(KK))))((S(K(S((SK)K))))K))abc
(c(a)(b))
SKI>

Setting the environment variable SKI_LOG=1 will show the derivations as they happen.

Examples

Examples of some birds:

(* kite returns second argument *)
{|KIab|} |> Sk.String.eval_print;
  (* prints: (b) *)

(* transform your expression into a lambda *)
{l|SIIa|l} 1 |> Sk.Int.eval_print;
  (* prints: (1(1)) *)

(* ornithology: cardinals and larks (and more) are defined in the Birds module*)
let open Birds in
  {|Cabc|} |> Sk.String.eval_print;
    (* prints: (ac(b)) *)

  {|Lab|} |> Sk.String.eval_print
    (* prints: (a(b(b))) *)

The curious can set the environment variable SKI_LOG=1 here as well to see what's going on under the hood.

References

  • To learn about ppx extensions, I followed this nice blog and referenced the source when in need of documentation.
  • Implementing this would have been a pain without the ppx_tools.