[Haskell-cafe] Finding the average in constant space

Steffen Schuldenzucker sschuldenzucker at uni-bonn.de
Sun May 27 21:33:46 CEST 2012

Hi Chris,

On 05/27/2012 10:04 AM, Chris Wong wrote:
> I just came up with a way of executing multiple folds in a single
> pass. In short, we can write code like this:
>     average = foldLeft $ (/)<$>  sumF<*>  lengthF
> and it will only traverse the input list once.
> The code is at: https://gist.github.com/2802644
> My question is: has anyone done this already? If not, I might release
> this on Hackage -- it seems quite useful.

This is (a special case of) the main point in the design of iteratees. 
See e.g. the definition of the 'Iteratee' type in the enumeratee 
library. - Looks pretty much like your 'Fold' type with an additional 
state (done or not yet done).

Also, the pipe package seems to provide something similar.

-- Steffen

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