Hi, I sometimes strumble on the same quiestion that forces me to insert functions that process objects of a certain class inside their class definition. This occurs when a computation uses the object internally, neiter as parameter or as a return value or in the case of existential types. An example of the first: class Example a where irec :: IO a pr :: a → IO String sample2 :: a → IO () sample2 _ = do x ← irec :: IO a pr x return () sample :: Example a ⇒ a → IO () sample _ = do x ← irec :: IO a pr x return () With the flag -fglasgow-exts, the following error below appears in sample. without the flag, the error appears in both sample and sample2. I´m too lazy to find what concrete extension is involved and why, anyhow, in the case of sample, the compiler must generate a new type a1 with no context. Could not deduce (Example a1) from the context () arising from a use of `irec' at Control\Workflow\Users.hs:73:7-10 Possible fix: add (Example a1) to the context of an expression type signature In a stmt of a 'do' expression: x <- irec :: IO a In the expression: do x <- irec :: IO a pr x return ()
Alberto,
On Tue, Jan 12, 2010 at 7:17 PM, Alberto G. Corona
Hi,
I sometimes strumble on the same quiestion that forces me to insert functions that process objects of a certain class inside their class definition. This occurs when a computation uses the object internally, neiter as parameter or as a return value or in the case of existential types. An example of the first:
class Example a where irec :: IO a pr :: a → IO String sample2 :: a → IO () sample2 _ = do x ← irec :: IO a pr x return ()
sample :: Example a ⇒ a → IO () sample _ = do x ← irec :: IO a pr x return ()
With the flag -fglasgow-exts, the following error below appears in sample. without the flag, the error appears in both sample and sample2. I´m too lazy to find what concrete extension is involved and why, anyhow, in the case of sample, the compiler must generate a new type a1 with no context.
Could not deduce (Example a1) from the context () arising from a use of `irec' at Control\Workflow\Users.hs:73:7-10 Possible fix: add (Example a1) to the context of an expression type signature In a stmt of a 'do' expression: x <- irec :: IO a In the expression: do x <- irec :: IO a pr x return ()
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In the code for `sample', you give a type signature to x, `IO a'. However, the `a' there is different from the `a' in the signature for `sample'. Perhaps ScopedTypeVariables http://www.haskell.org/haskellwiki/ScopedTypeVariables will help? Sincerely, Brad
Am Mittwoch 13 Januar 2010 01:17:17 schrieb Alberto G. Corona:
Hi,
I sometimes strumble on the same quiestion that forces me to insert functions that process objects of a certain class inside their class definition. This occurs when a computation uses the object internally, neiter as parameter or as a return value or in the case of existential types. An example of the first:
class Example a where irec :: IO a pr :: a → IO String sample2 :: a → IO () sample2 _ = do x ← irec :: IO a pr x return ()
sample :: Example a ⇒ a → IO () sample _ = do x ← irec :: IO a pr x return ()
With the flag -fglasgow-exts, the following error below appears in sample. without the flag, the error appears in both sample and sample2. I´m too lazy to find what concrete extension is involved and why,
{-# LANGUAGE ScopedTypeVariables #-} sample :: forall a. Example a => a -> IO () sample _ = do x <- irec :: IO a pr x return () Unless you bring the type variable a into scope, the 'a' in the signature of irec within sample is a fresh type variable, so it looks to the compiler like sample :: Example a => a -> IO () sample _ = do x <- irec :: IO b pr x return () You can make it without language extensions: sample :: Example a => a -> IO () sample dummy = do x <- irec pr (x `asTypeOf` dummy) return ()
anyhow, in the case of sample, the compiler must generate a new type a1 with no context.
Could not deduce (Example a1) from the context () arising from a use of `irec' at Control\Workflow\Users.hs:73:7-10 Possible fix: add (Example a1) to the context of an expression type signature In a stmt of a 'do' expression: x <- irec :: IO a In the expression: do x <- irec :: IO a pr x return ()
participants (3)
-
Alberto G. Corona -
Brad Larsen -
Daniel Fischer