[Haskell-cafe] ghc 7.0.3 view patterns and exhaustiveness
Twan van Laarhoven
twanvl at gmail.com
Wed Sep 21 23:57:16 CEST 2011
On 2011-09-21 22:06, Brent Yorgey wrote:
> On Tue, Sep 20, 2011 at 10:31:58PM -0400, Richard Cobbe wrote:
>> numVarRefs :: Term -> Integer
>> numVarRefs (view -> Var _) = 1
>> numVarRefs (view -> App rator rand) = numVarRefs rator + numVarRefs rand
>> numVarRefs (view -> Lam _ body) = numVarRefs body
>> -- numVarRefs (view -> _) = error "bogus"
> This is a known limitation. Your particular example is perhaps not so
> hard to figure out, but what if we had
> view :: Bool -> Bool
> view x = search for a counterexample to the Goldbach conjecture; if
> one is found, return x, otherwise return False
> foo (view -> False) = ...
But in Richard's example, all possible values of the result of view are handled.
For your foo example, the equivalent would be.
foo (view -> False) = ...
foo (view -> True) = ...
Ghc should be able to detect this case, and not issue a warning. All that needs
to be done is check if the function can be rewritten to
numVarRefs t = case view t of ...
Where ... is exhaustive.
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