[Haskell-cafe] non-exhaustive pattern match(es)?

[Semen Trygubenko / Семен Тригубенко]

Niklas and Chris:

On Wed, Jun 11, 2014 at 11:26:57AM +1200, Chris Wong wrote:
> The problem is in these lines:
> 
> >     | i <  i'   = []
> >     | i == i'   = []
> >     | i >  i'   = []

Thank you for your replies — it all makes sense now!

I arrived at the solution (addition of otherwise) by trial and error;
the pattern in the last definition, namely,

i (c :cs) ((i',t) :os)

looks pretty exhaustive, so when the compiler issued a warning, proposing
the addition of

_ (_ : _) ((_, _) : _)

, I scratched my head a bit before realizing the problem was not with the
pattern, but with the comparisons further down, in the guards. And, of course,
"patterns not matched" included many patterns that _were_ matched (in this case,
everything :)). Wouldn't it be great if it was something like

Patterns not matched: _ (_ : _) ((_, _) : _) | otherwise =

instead?



On Wed, Jun 11, 2014 at 12:17:49AM +0100, Semen Trygubenko / Семен Тригубенко wrote:
> > example.hs:2:1: Warning:
> >     Pattern match(es) are non-exhaustive
> >     In an equation for `f':
> >         Patterns not matched: _ (_ : _) ((_, _) : _)

-- 
Семен Тригубенко http://trygub.com
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