Erlang pattern matching with functions

Question

As Erlang is an almost pure functional programming language, I'd imagine this was possible:

case X of
    foo(Z) -> ...
end.

where foo(Z) is a decidable-invertible pure (side-effect free) bijective function, e.g.:

foo(input) -> output.

Then, in the case that X = output, Z would match as input.

Is it possible to use such semantics, with or without other syntax than my example, in Erlang?

Answer 1

No, what you want is not possible.

To do something like this you would need to be able to find the inverse of any bijective function, which is obviously undecidable.

Answer 2

I guess the reason why that is not allowed is that you want to guarantee the lack of side effects. Given the following structure:

case Expr of
    Pattern1 [when GuardSeq1] ->
        Body1;
    ...;
    PatternN [when GuardSeqN] ->
        BodyN
end

After you evaluate Expr, the patterns are sequentially matched against the result of Expr. Imagine your foo/1 function contains a side effect (e.g. it sends a message):

foo(input) ->
  some_process ! some_msg,
  output.

Even if the first pattern wouldn't match, you would have sent the message anyway and you couldn't recover from that situation.