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functionerlangpattern-matching
Pindatjuh
Pindatjuh

Reputation: 10526

Erlang pattern matching with functions

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?

Upvotes: 5

Views: 647

Answers (4)

Roberto Aloi
Roberto Aloi

Reputation: 30985

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.

Upvotes: 2

rvirding
rvirding

Reputation: 20916

No, Erlang only supports literal patterns!

And your original request is not an easy one. Just because there is a an inverse doesn't mean that it is easy to find. Practically it would that the compiler would have to make two versions of functions.

Upvotes: 1

tamasd
tamasd

Reputation: 5913

What you can do is:

Y = foo(Z),
case X of
  Y -> ...
end.

Upvotes: 1

sepp2k
sepp2k

Reputation: 370102

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.

Upvotes: 4

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