[mercury-users] Switches with an "else" branch?

[Ondrej Bojar]

Hi.

I'm trying to write deterministic switches but cover many cases in one
branch:

:- type my_type ---> one; two; three; four.

:- pred my_pred(my_type::in) is det.

my_pred(one).
my_pred(two).
my_pred(Else) :-
  Else \= one, Else \= two.

The Mercury compiler doesn't infer that all cases are covered (exactly
once) and claims:

testdetswitch.m:008: In `my_pred(in)':
testdetswitch.m:008:   error: determinism declaration not satisfied.
testdetswitch.m:008:   Declared `det', inferred `semidet'.
testdetswitch.m:010:   The switch on HeadVar__1 does not cover
                        testdetswitch:four/0 and/or testdetswitch:three/0.
testdetswitch.m:013:   Negated goal can succeed.
testdetswitch.m:013:   Negated goal can succeed.
testdetswitch.m:010:   Disjunction has multiple clauses with solutions.

Is nested if-then-else the only way to write such "switches with default"?

Thanks, Andrew.

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