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

Andreas Abel andreas.abel at ifi.lmu.de
Fri Jul 20 10:02:48 CEST 2012

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...


On 17.07.12 10:46 AM, oleg at 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.abel at ifi.lmu.de

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