[Haskell-beginners] Issue with lazy eval + trace

[Hugo Ferreira]

On 11/07/2011 01:47 PM, Daniel Fischer wrote:
> On Monday 07 November 2011, 09:41:24, Hugo Ferreira wrote:
>> Hello,
>>
>> I am using "import Debug.Trace" to debug some code but it is
>> not working as expected. I believe this is due to lazy evaluation.
>> But I cannot see how I can circumvent it. To exemplify I have
>> the following function:
>>
>> tagFun :: (POSTags ->  Tag) ->  POSTags ->  POSTags
>> tagFun f corpus = Z.foldlz' tag Z.empty corpus
>>     where
>>         tag acc z = Z.insert newTag acc
>>          where
>>            (token, correctTag, _proposdTag) = trace "cursor " Z.cursor z
>>            proposed = trace "proposed tag" (f z)
>>            newTag = ( token, correctTag, proposed )
>>
>>
>> If I place tracing at:
>>
>>         tag acc z = trace ("tag "++token++" = "++ show newTag) Z.insert
>> newTag acc
>>
>> then it works ok. However the lines
>>
>>            (token, correctTag, _proposdTag) = trace "cursor " Z.cursor z
>>            proposed = trace "proposed tag" (f z)
>>
>> don't work. They only appear once. I assume they should appear with
>> every element in the zipper. Can anyone explain why?
>
> You're compiling with optimisations, aren't you?

Not that I am aware of. I did not set up any compilation flags in the
cabal file.

> You need the trace strings to depend on the input, otherwise the optimiser
> sees no reason to repeat the work. (The constant part is lifted out of the
> lambda, a consequence of unsafePerformIO.)
>

Ok, I understand now. Should have appended some output to the trace
output string (thats what the working trace does).

> The line `trace "cursor " Z.cursor z' actually shouldn't print multiple
> traces even without optimisations, since it's
>
> (trace "cursor " Z.cursor) z
>

Ooops, thats a bug 8-(

> so the trace applies to the evaluation of the function Z.cursor, which
> should only be done once.
>

Ok, think I got it.

Thank you,
Hugo F.