[Haskell-cafe] Use unsafePerformIO to catch Exception?

Luke Palmer lrpalmer at gmail.com
Thu Mar 26 21:01:28 EDT 2009

On Thu, Mar 26, 2009 at 6:42 PM, Alexander Dunlap <
alexander.dunlap at gmail.com> wrote:

> On Thu, Mar 26, 2009 at 5:23 PM, wren ng thornton <wren at freegeek.org>
> wrote:
> > It's a static error, detectable statically, and yet it's deferred to the
> > runtime. I'd much rather the compiler catch my errors than needing to
> create
> > an extensive debugging suite and running it after compilation. Is this
> not
> > the promise of purity?
> 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

(\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.

Much as we would like to pressure the language to support static
constraints, I don't think we are yet in a position to.  It's not the kind
of thing you can just throw on and be done with it; see Conor McBride's
current project for an example of the issues involved.

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