[m-dev.] Adding default clauses to the language.

[Ralph Becket]

The following is a fairly common idiom in Mercury:

:- func f(string) = int.

f(S) = ( if f_2(S) = X then X else throw(...) ).

:- func f_2(string) = int is semidet.

f_2("foo") = 1.
f_2("bar") = 2.
f_3("baz") = 3.

Would it be a terrible thing to add the option of having
default clauses to the language, so one might write

:- func f(string) = int.

f("foo") = 1.
f("bar") = 2.
f("baz") = 3.
otherwise f(_) = throw(...).

Similarly, one might shorten det_map2 to

det_map2(_, [],       []      ) = [].
det_map2(F, [X | Xs], [Y | Ys]) = [F(X, Y) | det_map2(F, Xs, Ys)].

otherwise det_map2(_, _, _) = throw("inputs of different lengths").

rather than having to duplicate the throw for the (F, [], [_|_])
and (F, [_|_], []) cases.  It's not such a big deal for det_map2,
but having to catch each possible error case for det_map3 would
be real hassle; the programmer instead backs off to an if-then-else
which is less clear IMHO:

det_map3(F, Xs0, Ys0, Zs0) =
    ( if      Xs0 = [], Ys0 = [], Zs0 = []
      then    []
      else if Xs0 = [X | Xs], Ys0 = [Y | Ys], Zs0 = [Z | Zs]
      then    [F(X, Y, Z) | det_map3(F, Xs, Ys, Zs)]
      else    throw(...)
    ).

vs

det_map3(_, [],       [],       []      ) = [].
det_map3(F, [X | Xs], [Y | Ys], [Z | Zs]) = [F(X, Y, Z) | det_map3(Xs,
Ys, Zs)].

otherwise det_map3(_, _, _, _) = throw(...).

Thoughts?

- Ralph
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