Re: Error when ($) is used, but no error without

27 Apr 2006

      Hello Robin,

Thursday, April 27, 2006, 8:01:16 AM, you wrote:
...
g x = f $ x  -- the only change
...
gives this error:
...
Inferred type is less polymorphic than expected
       Quantified type variable `a' escapes
       Expected type: a a1 -> b
       Inferred type: C a1 -> Int
     In the first argument of `($)', namely `f'
     In the definition of `g': g x = f $ x

...
What's going on here?
may be, because '$' can't work with rank-2 types (at least error
message is the same as i got when tried to use "runST $ ..."). i asked
similar question some time ago, below are the answers (you can find
them on pipermail):

Bulat Ziganshin wrote:

the following code can't go through typechecking
...
import Control.Monad.ST
import Data.Array.ST
main = print $ runST $
           do arr <- newArray (1,10) 127
              a <- readArray arr 1
              writeArray arr 1 216
              b <- readArray arr 1
              return (a,b)
Indeed. The short answer: use 
        runST (long expression) 
rather than
        runST $ long expression 

when it comes to higher-ranked functions such as runST.
A longer answer:
        http://www.haskell.org/pipermail/haskell-cafe/2004-December/008062.html
...
let me know what i need to read to fix it myself
MLF (see Daan Leijen, A. Loeh, `Qualified types for MLF', ICFP05)
Bulat Ziganshin wrote:

... for the same reason as this one doesn't get through:

import Control.Monad.ST
import Data.Array.ST
main = print $ runST $
           do return ()

... but this one does:

import Control.Monad.ST
import Data.Array.ST
main = print $ runST (
           do return ())

it's all about rank-2 types; see SPJ's et al. paper on type inference
for these types. However, I guess that the jury is still out, say this
specific rank-2 behavior may be revised (and I also hope so).

HTH
Ralf
...
-----Original Message-----
From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-
bounces@haskell.org] On Behalf Of Bulat Ziganshin
Sent: Tuesday, January 03, 2006 2:28 AM
To: haskell-cafe@haskell.org
Subject: [Haskell-cafe] ST monad
Hello
the following code can't go through typechecking. can anyone help me
to fix it or, better, let me know what i need to read to fix it
myself? :)
import Control.Monad.ST
import Data.Array.ST
main = print $ runST $
           do arr <- newArray (1,10) 127
              a <- readArray arr 1
              writeArray arr 1 216
              b <- readArray arr 1
              return (a,b)
PS: error message is
b.hs:4:15:
    Inferred type is less polymorphic than expected
      Quantified type variable `s' escapes
      Expected type: ST s a -> b
      Inferred type: (forall s1. ST s1 a) -> a
    In the first argument of `($)', namely `runST'
    In the second argument of `($)', namely
        `runST
         $ (do
              arr <- newArray (1, 10) 127
              a <- readArray arr 1
              writeArray arr 1 216
              b <- readArray arr 1
              return (a, b))'
-- 
Best regards,
 Bulat                            mailto:Bulat.Ziganshin@gmail.com