[Haskell-beginners] deep seq and bang patterns

[Emmanuel Touzery]

to be clear: I definitely have strict evaluation now. It works. And it
helped me to fix my bug (it's fixed now).

But I think that to actually get strict evaluation I needed BOTH bang
patterns AND deep seq, at that same time... Which seems strange to me, I
would think that either would suffice.

emmanuel


On Tue, Dec 25, 2012 at 1:17 PM, Emmanuel Touzery <etouzery at gmail.com>wrote:

> Hello,
>
>  laziness just gave me some headache now. Because some of my code was
> evaluated much later, well first the memory use exploded but also I got an
> error which was really difficult to understand because it manifested much
> later than I would intuitively expect (when I read the record field, not
> when I wrote it...).
>
>  Now, if nothing else for space usage needs, I need to make this strict.
>
>  At first I tried bang patterns on my record:
>
> {-# LANGUAGE BangPatterns #-}
>
> data TvShow = TvShow
>     {
>         channel :: Channel,
>         title :: !T.Text,
>         startTime :: !T.Text,
>         summary :: !T.Text
>     }
>     deriving (Eq, Show)
>
>  That did not help.
>
>  Then I tried deepseq:
>
> instance NFData TvShow
>
>     return $!! result
>
>  That did not help.
>
>  And here's the catch: doing BOTH helps... I mean I think I'll check it
> again because I find it hard to believe but right now it really seems it
> behaves like that...
>
>  Is that possible? That I need to combine BOTH deepseq and bang patterns
> to actually get my code to evaluate when filling in the data? Or am I going
> crazy?
>
>  Thank you!
>
> Emmanuel
>
>
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