[erlang-questions] This clause cannot match because....

[Stefan Hellkvist]

Hi all,

I was just struck by a bug in my code due to wrong ordering of clauses.
Given a function f/1 which could be simplified to something like this:

f([a, B]) ->
    B;
f([a, b]) ->
    will_never_happen.

The problem is that the clause f([a, b]) is never taken simply because it
is fully covered by the first clause which matches all values for the
second element in the list. If I had put the second clause before the more
general clause it would have worked the way I thought it should.

I'm curious though. Why did the compiler not spot the problem in the code
for f/1 when it can obviously spot the problem in a, to me at least,
similar function f/2 seen below?:

f(a, B) ->
    B;
f(a, b) ->
    will_never_happen.

In this example, (f/2), I get a compilation error with a very good and
detailed description of what the problem is, but in the first case (f/1)
the compiler does not see the problem (or cannot?) with the covering
clause.

Is this some edge case where the compiler has just stopped caring and I
must use dialyser to spot the problem at an earlier stage than at runtime?

Stefan


p.s. funny thing is that if I use a tuple instead of a list to create f/1
in the same way the compiler is once again able to spot the problem, but
not with a list as argument.
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