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

[Ralph Becket]

Ah well, it would have been nice had it been workable :)

Could some approximation be useful with, say, some mechanism
for throwing exceptions for cases that aren't covered,
regardless of mode?

E.g.

:- func (func(T1, T2, T3) = T, list(T1), list(T2), list(T3)) = list(T)
    is promised_det.
       ^^^^^^^^^^^^

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

where promised_det is the same as det, except that an exception is
raised should the function fail (i.e. it converts a semidet definition
into one that is det.)  Does moving the new syntax into the mode
declaration do us any good?

This doesn't provide the ability to have a non-exception throwing 
catch-all mechanism, but does handle the common "this really should be
exhaustive; throw an exception if I'm wrong" situation.

You see often partial pattern matchings in Haskell & ML code, and they
are clearer for not having to explicitly list the exception cases.

- Ralph
--------------------------------------------------------------------------
mercury-developers mailing list
Post messages to:       mercury-developers at cs.mu.oz.au
Administrative Queries: owner-mercury-developers at cs.mu.oz.au
Subscriptions:          mercury-developers-request at cs.mu.oz.au
--------------------------------------------------------------------------