RE: [Haskell-cafe] Monotype error
It's a poor error message, but GHC's entire handling of impredicative
polymorphism is poor at the moment. Indeed, I'm seriously considering removing
it altogether until we can come up with a more robust story. (So don't rely on
it!)
The error happens because you are trying to use the type (forall a. a) in a
context that requires a monotype (one with no foralls). I have not stared at
the typing rules (in our papers) to convince myself that your program does
transgress them; indeed, I regard the necessity to do so as evidence that the
approach is not robust.
Simon
| -Original Message-
| From: haskell-cafe-boun...@haskell.org
[mailto:haskell-cafe-boun...@haskell.org] On
| Behalf Of Martijn van Steenbergen
| Sent: 14 October 2009 19:35
| To: Haskell Cafe
| Subject: [Haskell-cafe] Monotype error
|
| Dear café,
|
| {-# LANGUAGE Rank2Types #-}
| {-# LANGUAGE ImpredicativeTypes #-}
|
| type Void = forall a. a
|
| newtype Mono a = Mono { runMono :: [Void] }
|
| beep :: Mono a - Mono a
| beep (Mono vs) = Mono (map undefined vs)
|
| Compiling this with GHC results in:
|
| Monotype.hs:9:28:
| Cannot match a monotype with `Void'
|Expected type: Void
|Inferred type: a
|
| What does this error mean and why does the code not compile?
|
| Thanks!
|
| Martijn.
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Re: [Haskell-cafe] Monotype error
See our previous discussion on this topic here:
http://www.nabble.com/Fwd:-Unification-for-rank-N-types-td23942179.html
Thanks,
Vladimir
On Wed, Oct 14, 2009 at 10:35 PM, Martijn van Steenbergen
mart...@van.steenbergen.nl wrote:
Dear café,
{-# LANGUAGE Rank2Types #-}
{-# LANGUAGE ImpredicativeTypes #-}
type Void = forall a. a
newtype Mono a = Mono { runMono :: [Void] }
beep :: Mono a - Mono a
beep (Mono vs) = Mono (map undefined vs)
Compiling this with GHC results in:
Monotype.hs:9:28:
Cannot match a monotype with `Void'
Expected type: Void
Inferred type: a
What does this error mean and why does the code not compile?
Thanks!
Martijn.
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[Haskell-cafe] Monotype error
Dear café,
{-# LANGUAGE Rank2Types #-}
{-# LANGUAGE ImpredicativeTypes #-}
type Void = forall a. a
newtype Mono a = Mono { runMono :: [Void] }
beep :: Mono a - Mono a
beep (Mono vs) = Mono (map undefined vs)
Compiling this with GHC results in:
Monotype.hs:9:28:
Cannot match a monotype with `Void'
Expected type: Void
Inferred type: a
What does this error mean and why does the code not compile?
Thanks!
Martijn.
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Re: [Haskell-cafe] Monotype error
* Martijn van Steenbergen mart...@van.steenbergen.nl [2009-10-14
20:35:06+0200]
Dear café,
{-# LANGUAGE Rank2Types #-}
{-# LANGUAGE ImpredicativeTypes #-}
type Void = forall a. a
newtype Mono a = Mono { runMono :: [Void] }
beep :: Mono a - Mono a
beep (Mono vs) = Mono (map undefined vs)
Compiling this with GHC results in:
Monotype.hs:9:28:
Cannot match a monotype with `Void'
Expected type: Void
Inferred type: a
What does this error mean and why does the code not compile?
It works if you annotate the type of undefined:
beep (Mono vs) = Mono (map (undefined :: Void - Void) vs)
--
Roman I. Cheplyaka :: http://ro-che.info/
Don't let school get in the way of your education. - Mark Twain
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