Not enough strictness in toListM

[Gregory Schwartz]

Using toListM when large files are used results in stack space overflows. When replaced with this (albeit horrible) definition,

P.fold go [] id
  where
    go acc []  = acc
    go acc [x] = x : acc

there is no more stack space overflow. A fix would very much be appreciated as there is a lot of use to get out of toListM in my case!

[gabby]

Yeah, it's totally okay to use toListM if you really need it.

Also, there's nothing ugly about what you wrote (there are slight errors, but I recognize this diff-list trick). I use this pattern all the time in my own code when building a list. In fact, what you wrote is basically equivalent to:

import qualified Control.Foldl (purely, list)
import qualified Pipes.Prelude (fold)

toListM = purely fold list

... since the list fold from the foldl library uses the same diff-list trick to assemble the intermediate list.

Could you contribute a pull request for this? You don't need to use foldl, just use what you wrote.

[Gregory Schwartz]

I should mention, to do this trick I needed P.map (: []) beforehand.

[gabby]

Actually, you can just write:

toListM = Pipes.fold step begin done
  where
    step x a = x . (a:)
    begin = id
    done x = x []

This is the "diff-list" trick that I was referring to.

[gabby]

I'll mark this closed now that your pull request is merged