Match ocaml versions with Reasonml with `opam`

This week I was trying to write a ppx in ocaml use it in my reasonml project but I was getting an error about mismatching Ocaml versions.

Ast_mapper: OCaml version mismatch or malformed input

When I checked my versions sure enough there was a mismatch:

> bsc
BuckleScript 4.0.0 (Using OCaml4.02.3+BS )
> ocaml -version
The OCaml toplevel, version 4.07

Reasonml and Bucklescript use an older version of ocaml, version 4.02.3. I had compiled my ppx binary however with 4.07.

Fortunately it is rather easy to switch ocaml versions using opam, the Ocaml package manager. If you have opam installed (on mac brew install opam) then you can use the switch command.

opam switch 4.02.3

Running opam switch without an argument will show you all the available versions.

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