Erlang : Returning from a function

Question

I have a function in which I have a series of individual case statements.

case ... of
     ...
end,

case ... of
     ...
end,

...

etc.

I want to return from the function immediately when a particular case condition occurs in one of the case statements - so that the next case statement is not checked, and the function just exits/returns. How do I do that?

Answer 1

Erlang does not have a return operator. You will need to refactor your code into smaller functions.

Your original code has two case expressions chained with the comma operator. I presume you have some side effects in the first case expression that you want to preserve. Below, I'm using an imaginary return operator:

case ... of
  P1 -> return E1;
  P2 -> E2;
end,

case ... of
  ...
end

An expression like this can be converted to real Erlang code using small functions and pattern matching with something resembling this:

case1(P1, ...) -> E1;
case1(P2, ...) -> E2, case2(...).
case2(...) -> ...

Disclaimer: It's been 10 years since I've written Erlang code, so my syntax may be off.

Answer 2

In Erlang you just use the pattern matching to trigger the appropriate function. If you have too many clauses to cover and deal with I would also suggest to refactor the code a little bit.

Answer 3

I would suggest you refactor to harness the full power of Erlang and its pattern matching abilities.

There isn't a return operator. Also, a little known fact is you can do something like:

Return=case ... of

a case statement can have a "return" value.

Answer 4

use catch/throw

The caller says:

X = (catch foo(A, B)).

then write

foo(A, B) ->
    case ... of
     ...throw(X) ..
    end,

    case ... of
     ... throw (Y)
    end,
    ...

This is generally considered poor programming practice - since the program has multiple exit points and is difficult to grock

Answer 5

Pattern matching is a good way to refactor a case statement - you can do something like this

testcase(1, X, Y) -> .... passed1;
testcase(2, X, Y) -> .... passed2;
testcase(N, X, Y) when N > 2 -> .... passedlarge;
testcase(_, X, Y) -> defaultcase.

and then your case statement simply wraps up to:

X = testcase(Number, Param1, Param2).

(X will be either passed1, passed2, passedlarge or defaultcase in this contrived example)

Answer 6

One way is to cascade your case statements:

my_fun(X) ->
  case cond1(X) of
    true -> ret1;
    _ ->
      case cond2(X) of
        true -> ret2;
        _ ->
          ...
      end
  end.

Another one is to separate your case statements into clauses:

my_fun(X) ->
  my_fun(cond1, X).

my_fun(cond1, X) ->
  case cond1(X) of
    true -> ret1;
    _    -> my_fun(cond2, X)
  end;

my_fun(cond2, X) ->
  case cond2(X) of
    true -> ret2;
    _    -> my_fun(cond3, X)
  end;

...