Re: [Haskell-cafe] trace function

2006-07-20 Thread Malcolm Wallace 
Alexander Vodomerov <[EMAIL PROTECTED]> wrote:

> main = do
>   putStrLn "xxx"
>   return (trace "yyy" ())
>   putStrLn "zzz"
> 
> only xxx and zzz is displayed. yyy is missing.

This is because you never demanded the value of (trace "yyy" ()), so it
was never computed.  The joys of laziness!  To force its evaluation try

  main = do
putStrLn "xxx"
() <- return (trace "yyy" ())
putStrLn "zzz"

Regards,
Malcolm
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Re: [Haskell-cafe] trace function

2006-07-20 Thread Neil Mitchell 

Hi,

Either one of these will work:

main = do
putStrLn "xxx"
x <- return (trace "yyy" ())
x `seq` putStrLn "zzz"

main = do
putStrLn "xxx"
trace "yyy" (return ())
putStrLn "zzz"


This works fine, the problem is that trace is defined to output the
first parameter before returning the second. In the case of return ()
the () is never evaluated, hence the item is never generated, and the
trace is not fired. Adding a seq to the result gets the trace going.

This just shows how fragile trace is! However, if you use it in a real
situation, rather than just a play one like this, it will probably
work for you.

Thanks

Neil
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[Haskell-cafe] trace function

2006-07-20 Thread tpledger 
Alexander Vodomerov wrote:
> import Debug.Trace
>
> main = do
>   putStrLn "xxx"
>   return (trace "yyy" ())
>   putStrLn "zzz"
>
> only xxx and zzz is displayed. yyy is missing.
> Why trace is not working?

Nothing uses the value of (trace "yyy" ()), so it is never
evaluated.

Try this instead, which uses the value for a pattern match:

() <- return (trace "yyy" ())

Or this, which makes the trace part of the sequence of IO
actions:

trace "yyy" (return ())

HTH,
Tom
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[Haskell-cafe] trace function

2006-07-20 Thread Alexander Vodomerov 
  Hello!

The function trace is supposed to write debug messages to console.
However, when I trying to run the following program

import Debug.Trace

main = do
  putStrLn "xxx"
  return (trace "yyy" ())
  putStrLn "zzz"

only xxx and zzz is displayed. yyy is missing.
Why trace is not working?

PS. GHC-6.4.2, Debian Linux on x86.

With best regards,
   Alexander.
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