[mercury-users] false function

[Richard A. O'Keefe]

There's one thing badly wrong with

	foo(X) =
	   ( if bar(X) then
	      value1
	   else if baz(X) then
	      value2
	   else
	      false
	   ).
	
which is that the value of the function is _not_ "false", there is no value
in that case.  I've seen \Omega (an actual Omega) used for this purpose and
(error "string") in a language without "semidet functions" and _|_ (as an
approximation to the symbol for bottom).  One nice notation for an undefined
value goes back to BCPL: "?".  Perhaps the best notation would be simply to
drop the "else" branch and write

	foo(X) = (if bar(X) then value1
	     else if baz(X) then value2).

By the way, I don't suppose there's any possibility of "infix if" is there?

	foo(X) = (value1 if bar(X) else
	          value2 if baz(X)).

Thought not.  Pity.
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