[mercury-users] false function

[Michael Day]

> > :- func false = _ is failure.
> 
> I don't like this idea: false is a proposition, not a function.

A better name, then? A value that cannot be computed? nonexistent?  
nothing? priceless? worthless?

To clarify, it is convenient for this kind of function:

foo(X) =
   ( if bar(X) then
      value1
   else if baz(X) then
      value2
   else
      nothing
   ).

which cannot be written as a semidet switch, as either bar or baz could 
potentially succeed (although in reality usually only one could, but there 
is no way to tell the compiler this yet at present?).

Michael

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