[plt-scheme] implementation of foldl

From: Matthias Felleisen (matthias at ccs.neu.edu)
Date: Wed Apr 29 10:46:13 EDT 2009

The difference between foldr and foldl is a truly semantic  
difference. The latter uses the argument as an accumulator, which is  
a "stronger" design style than direct design (i.e. foldr). The  
difference is approximately like the one between strong induction and  

-- Matthias

On Apr 29, 2009, at 10:38 AM, keydana at gmx.de wrote:

> Hi Thomas,
> thanks for your answer.
>> Maybe the interface is simply designed this way because it matches  
>> the
>> signature of cons used as a list constructor naturally.
> That's right, I somehow was mislead by the graphic order of cons'  
> arguments, I thought Haskell's way was more intuitive, but it's the  
> inverse ...
> Might be interesting to know if this really was the reason it was  
> implemented like that!
> In general, I would be curious to know if the following picture of  
> fold usage in scheme would be approximately correct:
> - for small lists, if a function can be written with foldl and  
> foldlr, choose whichever
> - more functions can only be written with with foldr because they  
> need the empty list at the end
> - for very long lists, better use foldl because it uses tail calls
> Ciao,
> Sigrid
>> However I
>> don't see any big advantage of that design. On the contrary it makes
>> the implementation of the variadic versions of fold and fold-right a
>> little more complicated to pass the initial / accumulator argument
>> last instead of first.
>> cu,
>> Thomas
>> -- 
>> When C++ is your hammer, every problem looks like your thumb.
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