[Haskell-cafe] a sort of chunk

[Viktor Dukhovni]

On Mon, Jan 20, 2020 at 05:39:16AM -0500, Viktor Dukhovni wrote:

> > > src/Hkl/Projects/Sixs.hs:67:5-35: warning: [-Wincomplete-uni-patterns]
> > >     Pattern match(es) are non-exhaustive
> > >     In a pattern binding: Patterns not matched: []
> > >    |
> > > 67 |     cons1 !x ~(c:cs) = (x : c) : cs
> > >    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > 
> > If this is something which can not be reach, is it possible to explain to ghc how to avoid these warning ?
> 
> However, with "-Wincomplete-uni-patterns" I still get this one, but the
> irrefutable binding is there for a reason, stricness in the second
> argument has undesirable performance implications, we know that the list
> won't be empty.  So perhaps compile this code in a module that does not
> have that warning turned on.

Actually, the warning can be eliminated by writing:

    cons1 x cs = (x : head cs) : tail cs

which just as "unsafe" (we know that `cs` won't be empty, but the
compiler does not), but use of the partial functions 'head' and 'tail'
does not elicit a warning.  The `!x` is also redundant, we've already
evaluated the weight of `x`'.

-- 
    Viktor.