Re: [Haskell-cafe] Type error with Type families

17 Sep 2012

      The problem is that the function 'element' is ambiguous, for the reasons
MigMit pointed out.

The standard solution to this problem is to add a dummy argument to fix the
type argument to the type function:

data Proxy a = Proxy

class ... => ReplaceOneOf full where
    type Item full ::  *

    -- implementations can just ignore the first argument
    element :: Proxy full -> Item full -> [Item full] -> Bool

    replaceOneOf :: ...
        ...
        | element (Proxy :: Proxy full) x from = ...

Now the choice of which 'element' to use can be determined by the type of
the proxy.

  -- ryan

On Sun, Sep 16, 2012 at 4:05 AM, Marco Túlio Pimenta Gontijo <
marcotmarcot@gmail.com> wrote:
...
Hi.
I cannot make this program type check:
{-# LANGUAGE TypeFamilies, FlexibleContexts #-}
    import qualified Data.ListLike as LL
class LL.ListLike full (Item full) => ReplaceOneOf full where
      type Item full :: *
      replaceOneOf :: [Item full] -> full -> full -> full
      replaceOneOf from to list
        | LL.null list = list
        | x `element` from
          = LL.concat [to, replaceOneOf from to $ LL.dropWhile
(`element` from) xs]
        | otherwise = LL.cons x $ replaceOneOf from to xs
        where
          x = LL.head list
          xs = LL.tail list
      element :: Item full -> [Item full] -> Bool
The error message is:
Line 9: 1 error(s), 0 warning(s)
Could not deduce (Item full0 ~ Item full)
    from the context (ReplaceOneOf full)
      bound by the class declaration for `ReplaceOneOf'
      at /home/marcot/tmp/test_flymake.hs:(4,1)-(15,45)
    NB: `Item' is a type function, and may not be injective
    Expected type: [Item full0]
      Actual type: [Item full]
    In the second argument of `element', namely `from'
    In the expression: x `element` from
I have tried using asTypeOf, but it did not work:
{-# LANGUAGE TypeFamilies, FlexibleContexts, ScopedTypeVariables #-}
    import qualified Data.ListLike as LL
class LL.ListLike full (Item full) => ReplaceOneOf full where
      type Item full :: *
      replaceOneOf :: Item full -> [Item full] -> full -> full -> full
      replaceOneOf xt from to list
        | LL.null list = list
        | (x `asTypeOf` xt) `element` from
          = LL.concat [to, replaceOneOf xt from to $ LL.dropWhile
(`element` from) xs]
        | otherwise = LL.cons x $ replaceOneOf xt from to xs
        where
          x = LL.head list
          xs = LL.tail list
      element :: Item full -> [Item full] -> Bool
Can someone point me to a solution?
Greetings.
--
marcot
http://marcot.eti.br/
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Re: [Haskell-cafe] Type error with Type families

Ryan Ingram