[Haskell-beginners] case statement and guarded equations

[Rein Henrichs]

I'll also mention that GHC's exhaustiveness checker will mark the pattern
match as exhaustive (since EvenOdd must be either Even or Odd and both
cases are given) but warn about the two guards since it doesn't know that
they form a dichotomy. You can use `otherwise`, which is just another name
for True, to convince GHC that your guards are exhaustive.

On Fri, Jul 7, 2017 at 3:38 PM Rein Henrichs <rein.henrichs at gmail.com>
wrote:

> Quoting the report[1], "A boolean guard, g, is semantically equivalent to
> the pattern guard True <- g," which means the answer is "Yes". A boolean
> guard is equivalent to a pattern match. A predicate involving ==, however,
> introduces an Eq constraint that would not be required by pattern matching.
> For a properly equivalent guard, you need to write your predicates using
> pattern matching
>
> isEven Even = True
> isEven _ = False
>
> to avoid the spurious Eq constraint.
>
> [1]
> https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-460003.13
>
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