[Haskell-beginners] Doubts about functional programming paradigm

[Henk-Jan van Tuyl]

On Sat, 12 Dec 2015 01:56:48 +0100, Dimitri DeFigueiredo  
<defigueiredo at ucdavis.edu> wrote:

:
> Couldn't match expected type ‘r’ with actual type ‘COParseResult’
>        ‘r’ is a rigid type variable bound by
>            the type signature for
> getParseResult :: ParseResult r => String -> IO r
>              ...
>
> I have a PhD in computer science, but never really liked programming
> languages back then and somehow I never learned what a "rigid type
> variable" is.

See
   [Haskell-cafe] What is a rigid type variable?
   https://mail.haskell.org/pipermail/haskell-cafe/2008-June/044622.html

:
> But would anyone care to explain to a
> novice in a couple paragraphs why foldl (+) 0 [1..10^9] may take 10 Gigs
> of RAM to calculate?

The foldl builds up a very long expression and evaluates it after the last  
element of the list is reached (the evaluation is non-strict, or lazy). If  
you use foldl' (from Data.List) instead, the calculation is done per  
element (the evaluation is strict).

For more details, see
   Foldr Foldl Foldl'
   https://wiki.haskell.org/Foldr_Foldl_Foldl'

   Lazy vs. non-strict
   https://wiki.haskell.org/Lazy_vs._non-strict

Regards,
Henk-Jan van Tuyl


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