Haha, I like this example.
However, if your are using ExtendedDefaultRules then you are likely to
know you are leaving the clean sound world of type inference. The
example of my student works on plain GHC...
The other responses to my message confined to point me to "best
practices" to avoid the design bugs of Haskell's type inference.
However, if I want "best practices", I can go to JavaScript.
To stirr action, mails on haskell-cafe seem useless.
I am already biased towards Haskell. But as an unbiased decision maker,
I would not consider a language with "undead code" for my projects...
Cheers,
Andreas
On 17.07.12 10:46 AM, o...@okmij.org wrote:
1. Haskell's type inference is NON-COMPOSITIONAL!
Yes, it is -- and there are many examples of it. Here is an example
which has nothing to do with MonomorphismRestriction or numeric
literals
{-# LANGUAGE #-}
class C a where
m :: a -> Int
instance C () where
m _ = 1
instance C Bool where
m _ = 2
main = do
x <- return undefined
let y = x
print . fst $ (m x, show x)
-- let dead = if False then not y else True
return ()
The program prints 1. If you uncomment the (really) dead code, it will
print 2. Why? The dead code doesn't even mention x, and it appears
after the printing! What is the role of show x, which doesn't do anything?
Exercises: what is the significance of the monadic bind to x? Why
can't we replace it with "let x = undefined"?
[Significant hint, don't look at it]
Such a behavior always occurs when we have HM polymorphism restriction
and some sort of non-parametricity -- coupled with default rules or
overlapping instances or some other ways of resolving overloading. All
these features are essential (type-classes are significant,
defaulting is part of the standard and is being used more and more).
--
Andreas Abel <>< Du bist der geliebte Mensch.
Theoretical Computer Science, University of Munich
Oettingenstr. 67, D-80538 Munich, GERMANY
andreas.a...@ifi.lmu.de
http://www2.tcs.ifi.lmu.de/~abel/
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
