[Haskell-cafe] Haskell's type inference considered harmful

[MigMit]

Actually, both examples show that the problem isn't type inference, it's defaulting mechanism.

Отправлено с iPhone

Jul 17, 2012, в 12:46, oleg at okmij.org написал(а):

> 
>>   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 ExtendedDefaultRules #-}
> 
> 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).
> 
> 
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