I tried to use ppx to autogenerate an sexp parser for a type that I define, and it compiles without errors, yet it gives runtime error when I actually run it by entering "(1 2)" as an sexp:
Fatal error: exception (Of_sexp_error "lib/a.ml.exp_of_sexp: unexpected variant constructor"
(invalid_sexp (1 2)))
Raised at Sexplib0__Sexp_conv.of_sexp_error in file "src/sexp_conv.ml", line 154, characters 30-72
The file a.ml contains the following type definition:
open Sexplib.Std
type var = string [@@deriving sexp]
type const =
| CNum of int
| CStr of string
| CBool of bool [@@deriving sexp]
type exp =
| E1
| E2
| EConst of const
| EVar of var
| EList of exp list [@@deriving sexp]
and the error shows up when I tried to call: exp_of_sexp (Core.Sexp.of_string (read_line ())) to read a line and parse it.
What exp value are you expecting the s-expression (1 2) to represent?
I think your code is fine, but your expectations are off.
I would expect input like (EVar "foo") or (EConst (CNum 42)) to parse just fine, though. To gain more insight, try printing string representations of the s-expressions generated by functions like sexp_of_exp to see what exp_of_sexp is looking for.