[Haskell-cafe] Powerset of a set

Olaf Klinke olf at aatal-apotheke.de
Tue Sep 15 13:02:48 UTC 2015

This was one of the most woundrous moments I ever had with reading a Haskell library: If lists are used to represent finite sets (hence disregarding order and multiplicity of elements), then 

Control.Monad.filterM (const [False,True]) :: [a] -> [[a]]

computes the powerset. Exercise for Jorge: Read the source code of filterM [*], understand what this function is doing, and why this indeed computes (a representation of) the powerset. 


[*] http://hackage.haskell.org/package/base-

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