Invalid syntax parsing error with PyreAst.Parser.Concrete.parse_expression

[jpham658]

Hi! I've been testing out pyre-ast as I'd like to use it in a project, and came across this behaviour:

If I call PyreAst.Parser.Concrete.parse_expression with "x = 4", I get an invalid syntax error, but the same does not occur when I use PyreAst.Parser.Concrete.parse_module. Why is this the case? I thought it would behave the same in both cases.

Code I used to test pyre-ast:

let example content =
  let open PyreAst.Parser in
  with_context (fun context ->
    (* replace parse_module here with parse_expression to get the error *)
  	match Concrete.parse_expression ~context content with
  	| Result.Error { Error.message; line; column; _ } ->
  	  let message = 
  	    Format.sprintf "Parsing error at line %d, column %d: %s"
  	    line column message
  	  in
  	  failwith message
  	| Result.Ok ast -> ast
  )

let () = 
  let ast = example "x = 4" in
  (* replace PyreAst.Concrete.Module with PyreAst.Concrete.Expression if you replace parse_module *)
  Sexplib0.Sexp.pp_hum Format.std_formatter (PyreAst.Concrete.Expression.sexp_of_t ast)

Error I received when parsing "x = 4" with parse_expression

Fatal error: exception Failure("Parsing error at line 1, column 3: invalid syntax")
Raised at Stdlib.failwith in file "stdlib.ml", line 29, characters 17-33
Called from Base__Exn.protectx in file "src/exn.ml", line 80, characters 8-11
Re-raised at Base__Exn.raise_with_original_backtrace in file "src/exn.ml" (inlined), line 60, characters 2-48
Called from Base__Exn.protectx in file "src/exn.ml", line 87, characters 13-49
Called from Dune__exe__Main in file "bin/main.ml", line 16, characters 12-27