ERL-1122: Function to fail in guards

[OTP-Maintainer]

Original reporter: hauleth
Affected version: Not Specified
Component: Not Specified
Migrated from: https://bugs.erlang.org/browse/ERL-1122

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Such function would be useful for custom guards in Elixir (and probably other languages that allow defining such). Example could be {{is_struct/2}} in Elixir which would fail if the second argument is non-atom. Current hack involves using function that will always fail, ex.:

{code:ruby}
defguard is_struct(struct, type)
  when is_map(struct) and (is_atom(type) or hd([]))
    and :erlang.map_get(:__struct__, struct) == type
{code}

However that seems pretty unidiomatic. Having function like {{erlang:guard_fail/0}} would make it more readable and would show the intended behaviour.

[OTP-Maintainer]

bjorn said:

There does not seem to be much use for a {{guard_fail/0}} BIF for Erlang programs, and for Elixir it seems that you could easily add your own {{guard_fail/0}} function that expands to an expression that will fail in a guard.

By the way, there is another way to make a guard fail. Perhaps this is clearer:
{code:java}
defguard is_struct(struct, type)
  when is_map(struct) and (is_atom(type) or :fail)
    and :erlang.map_get(:__struct__, struct) == type
{code}
This works because a non-boolean value as the left operand of any boolean operation ({{and}}, {{andalso}}, and so on) will cause the entire guard to fail.