[Haskell-beginners] mayBe stuck

Colin Paul Adams colin at colina.demon.co.uk
Fri Aug 6 04:40:05 EDT 2010

>>>>> "Brent" == Brent Yorgey <byorgey at seas.upenn.edu> writes:

    Brent> On Thu, Aug 05, 2010 at 06:20:06PM -0500, aditya siram wrote:
    >> Normally yes, but here we are guaranteed to get a 'Just ...'
    >> value because of the 'isNothing' guard.  -deech

    Brent> You are correct, but that's not the point.  Every time you
    Brent> use fromJust (or head, or unsafePerformIO...) you shift the
    Brent> burden of proving that it is safe from the compiler onto
    Brent> yourself.

But the compiler could indeed prove that it's safe, if the typing system
reflected the precondition. Since Haskell allows programming with
partial functions, you always have this burden at present.
Colin Adams
Preston Lancashire
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