[Haskell-cafe] Use unsafePerformIO to catch Exception?

wren ng thornton wren at freegeek.org
Thu Mar 26 21:37:14 EDT 2009

Luke Palmer wrote:
> Alexander Dunlap wrote:
> > Ultimately, it's not detectable statically, is it? Consider
> >
> > import Control.Applicative
> >
> > main = do
> >  f <- lines <$> readFile "foobar"
> >  print (head (head f))
> >
> > You can't know whether or not head will crash until runtime.
> Static checkers are usually conservative, so this would be rejected.  In
> fact, it's not always essential to depend on external information; eg. this
> program:
> (\x y -> y) (\x -> x x) (\x -> x)
> Behaves just like the identity function, so surely it should have type a ->
> a, but it is rejected because type checking is (and must be) conservative.
> Keeping constraints around that check that head is well-formed is a pretty
> hard thing to do.  Case expressions are easier to check for totality, but
> have the disadvantages that wren mentions.

My idea amounts to trying to make case expressions more first-class than 
they are now. As Luke says, we'd have to be conservative about it (until 
the dependent-types and total-functional-programming folks do the 
impossible), but I think there's still plenty of room for biting off a 
useful chunk of the domain without falling off that cliff.

Live well,

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