On Sunday 31 July 2011, 23:24:13, Alexey G wrote:

Hello. I have some problem with catching error.

My function try to read value from string and if it's fails - return default (d).

With this:

...

import Control.Exception readIOWith :: Read a => a -> String -> IO a readIOWith d x = catch tryRead errHandler

where errHandler :: SomeException -> IO a

There's your problem. The type variable 'a' in the local signature is completely unrelated to the type variable 'a' in the top-level signature (type variables have an implicit forall in Haskell), so the local signature says "errHandler can return `IO anything'", but it can of course only return `IO (Type of d)'. If you remove the signature, it should compile. (Another possibility is to bring the type variable into scope, that would require the ScopedTypeVariables language extension and an explicit forall.) However, this shouldn't need any IO at all, import Data.Char -- for isSpace readWithDefault :: Read a => a -> String -> a readWithDefault d s = case reads s of [(result,remaining)] | all isSpace remaining -> result _ -> d If you really need the IO return type: readIOWith d = return . readWithDefault d

...

errHandler _ = return d tryRead = readIO x

I have error:

Could not deduce (a ~ a1) from the context (Read a) bound by the type signature for readIOWith :: Read a => a -> String -> IO a at BWClub/Common/Helpers/Packets.hs:(19,1)-(22,33) `a' is a rigid type variable bound by the type signature for readIOWith :: Read a => a -> String -> IO a at BWClub/Common/Helpers/Packets.hs:19:1 `a1' is a rigid type variable bound by the type signature for errHandler :: SomeException -> IO a1 at BWClub/Common/Helpers/Packets.hs:21:11 In the first argument of `return', namely `d' In the expression: return d In an equation for `errHandler': errHandler _ = return d

But this code works:

...

readIOWith d x = catch tryRead (\e -> print (e :: SomeException) >> return

d)

...

where tryRead = readIO x

Here, there's no signature introducing a fresh type variable.

Sorry for my english.

The problem is it can't be sure that the a in readIOWith is the same as the a in errHandler based on how it is used because the errHandler version is ignored entirely in this case. There are two options: Remove the type signature of errHandler, and the compiler will infer it for you. Unfortunately that won't work in this case because the exception type is ambiguous, so you need the signature to make things works. The second option is add the ScopedTypeVariables extension to your code, then add a forall a. to the signature of readIOWith. The code will look like this: {-# LANGUAGE ScopedTypeVariables #-} import Control.Exception as E readIOWith :: forall a. Read a => a -> String -> IO a readIOWith d x = E.catch tryRead errHandler where errHandler :: SomeException -> IO a errHandler _ = return d tryRead = readIO x What this does is you are telling it that it should assume that all a's in any part of the function as the same type. The forall is normally implied but for some reason this extension requires that you specify it. Remember that if your function has several type variables in it, like a, b, and c, the forall needs to cover them all (forall a b c.). On Sun, Jul 31, 2011 at 5:24 PM, Alexey G wrote:

Hello. I have some problem with catching error. My function try to read value from string and if it's fails - return default (d). With this:

...

import Control.Exception readIOWith :: Read a => a -> String -> IO a readIOWith d x = catch tryRead errHandler    where errHandler :: SomeException -> IO a              errHandler _ = return d              tryRead        = readIO x I have error:

Could not deduce (a ~ a1)     from the context (Read a)       bound by the type signature for                  readIOWith :: Read a => a -> String -> IO a       at BWClub/Common/Helpers/Packets.hs:(19,1)-(22,33)       `a' is a rigid type variable bound by           the type signature for readIOWith :: Read a => a -> String -> IO a           at BWClub/Common/Helpers/Packets.hs:19:1       `a1' is a rigid type variable bound by            the type signature for errHandler :: SomeException -> IO a1            at BWClub/Common/Helpers/Packets.hs:21:11     In the first argument of `return', namely `d'     In the expression: return d     In an equation for `errHandler': errHandler _ = return d

But this code works:

...

readIOWith d x = catch tryRead (\e -> print (e :: SomeException) >> return d)    where tryRead      = readIO x Sorry for my english.

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