Erlang Meck: How does one mock only a specific function clause?

Question

Give a function with multiple clauses, I'd want to mock only a specific case and for every other input that would otherwise cause a 'function_clause' error, I'd want to have it handled by the original function. It's almost like a selective passthrough in erlang meck.

Answer 1

You need to use meck:passthrough/1: I created a module with a function like this:

-module(demo).
-export([original/1]).

original(1) -> one;
original(2) -> two;
original(3) -> three.

Then on the console…

1> meck:new(demo).
ok
2> meck:expect(demo, original,
2>             fun (1) -> not_one
2>               ; (Arg) -> meck:passthrough([Arg])
2>             end).
ok
3> demo:original(1).
not_one
4> demo:original(2).
two

Hope this helps :)