Re: [Haskell-cafe] Tracing Prelude.read exceptions

2011-12-11 Thread Malcolm Wallace 
I suggest switching from 'read' to a real parser that can give you proper error 
messages.  I use Text.Parse from the polyparse package, which is designed to 
parse back exactly the format produced by derived Show instances.  To derive 
the Parse class from your datatypes, the tool DRiFT is handy.

'runParser parse' will give you Either String a, where the string contains any 
error message.

Regards, Malcolm

On 11/12/2011, at 18:19, dokondr  wrote:

> Hi,
> I got quite used to a sequence providing simple data persistence :
> 1) Store my data to a file:
> writeFile fileName (show someData)
> 
> 2) Some time later read this data back:
> line <- readFile fileName
> let someData = read line :: SomeDataType
> 
> Having this done hundreds of times I now got stuck with step 2)  trying to 
> read moderately complex structure back. I get read exception in run-time:  
> fromList *** Exception: Prelude.read: no parse
> 
> I have checked and rechecked my types, data files, etc. - and still no idea.
> 
> So my question:
> Is there any way to trace Prelude.read exceptions to see exactly on what data 
> element read fails in run-time?
> 
> Thanks!
> 
>  
> 
> 
> 
> 
> 
> ___
> Haskell-Cafe mailing list
> Haskell-Cafe@haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe

___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe

Re: [Haskell-cafe] Tracing Prelude.read exceptions

2011-12-11 Thread Felipe Almeida Lessa 
On Sun, Dec 11, 2011 at 4:19 PM, dokondr  wrote:
> Hi,
> I got quite used to a sequence providing simple data persistence :
> 1) Store my data to a file:
> writeFile fileName (show someData)
>
> 2) Some time later read this data back:
> line <- readFile fileName
> let someData = read line :: SomeDataType

I can't help you with your question, but I suggest using safecopy [1]
even for simple cases.  It's *much* faster and also safer.  Also, it's
easy to use, just use

  L.writeFile fileName $ S.runPutLazy $ safePut someData

and

  ret <- S.runGetLazy safeGet <$> L.readFile fileName
  case ret of
Left err -> print err
Right someData -> ...

where

  import Control.Applicative ((<$>))
  import qualified Data.ByteString.Lazy as L -- [2]
  import qualified Data.Serialize.Get as S -- [3]
  import qualified Data.Serialize.Put as S -- [4]

Cheers,

[1] http://hackage.haskell.org/package/safecopy
[2] 
http://hackage.haskell.org/packages/archive/bytestring/0.9.2.0/doc/html/Data-ByteString-Lazy.html
[3] 
http://hackage.haskell.org/packages/archive/cereal/0.3.4.0/doc/html/Data-Serialize-Get.html
[4] 
http://hackage.haskell.org/packages/archive/cereal/0.3.4.0/doc/html/Data-Serialize-Put.html

-- 
Felipe.

___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe

[Haskell-cafe] Tracing Prelude.read exceptions

2011-12-11 Thread dokondr 
Hi,
I got quite used to a sequence providing simple data persistence :
1) Store my data to a file:
writeFile fileName (show someData)

2) Some time later read this data back:
line <- readFile fileName
let someData = read line :: SomeDataType

Having this done hundreds of times I now got stuck with step 2)  trying to
read moderately complex structure back. I get read exception in run-time:
fromList *** Exception: Prelude.read: no parse

I have checked and rechecked my types, data files, etc. - and still no idea.

So my question:
Is there any way to trace Prelude.read exceptions to see exactly on what
data element read fails in run-time?

Thanks!
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe