Module Ast_pattern

PPX rewriters often need to recognize fragments the OCaml AST, for instance to parse the payload of an attribute/expression. You can do that with a pattern matching and manual error reporting when the input is not what you expect but this has proven to quickly become extremely verbose and unreadable.

This module aims to help with that by providing first class AST patterns.

To understand how to use it, let's consider the example of ppx_inline_test. We want to recognize patterns of the form:

      let%test "name" = expr
    

Which is a syntactic sugar for:

      [%%test let "name" = expr]
    

If we wanted to write a function that recognizes the payload of %%test using normal pattern matching we would write:

      let match_payload = function
        | Pstr [ { pstr_desc = Pstr_value (Nonrecursive,
                                           [ { pvb_pat = Ppat_constant (Constant_string
                                                                          (name, None))
                                             ; pvb_expr = e
                                             ; _ } ])
                 ; _ } ] ->
          (name, e)
        | _ -> Location.raise_errorf ...
    

This is quite cumbersome, and this is still not right: this function drops all attributes without notice.

Now let's imagine we wanted to construct the payload instead, using Ast_builder one would write:

      let build_payload ~loc name expr =
        let (module B) = Ast_builder.with_loc loc in
        let open B in
        pstr [ pstr_value Nonrecursive (value_binding ~pat:(pstring name) ~expr) ]
    

Constructing a first class pattern is almost as simple as replacing Ast_builder by Ast_pattern:

      let payload_pattern name expr =
        let open Ast_pattern in
        pstr (pstr_value nonrecursive (value_binding ~pat:(pstring __) ~expr:__) ^:: nil)
    

Notice that the place-holders for name and expr have been replaced by __. The following pattern with have type:

 (payload, string -> expression -> 'a, 'a) Ast_pattern.t 

which means that it matches values of type payload and captures a string and expression from it. The two captured elements comes from the use of __.