[Haskell-cafe] foldl vs foldl'

Simon Richard Clarkstone simon.clarkstone at gmail.com
Sun Nov 9 19:24:37 EST 2008

Bas van Dijk wrote:
> On Wed, Nov 5, 2008 at 12:43 AM, Bas van Dijk <v.dijk.bas at gmail.com> wrote:
>> 2008/11/5 Daryoush Mehrtash <dmehrtash at gmail.com>:
>>> Are there cases (function or list) where the result of foldl (or foldr)would
>>> be different that foldl' (or foldr')?
>> Maybe this wiki article I wrote some time ago will answer your question:
>> http://haskell.org/haskellwiki/Foldr_Foldl_Foldl'
> Oops that link should be:
> http://haskell.org/haskellwiki/Foldr_Foldl_Foldl%27

I have an idea for the foldl diagram.  If you rotate the RHS 90deg 
clockwise, it slopes the same way as the original list, but the 'f's are 
still rotated sideways to signify which their inputs are.  That makes 
the relationship between imperative accumulator loops and foldl a bit 
clearer to me, as well as the relationship between the list and the 
recursion structure.


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