haskell - What is the point of pass and listen in Writer monad? -

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• how `pass` , `listen` work in writert? 4 answers

i have been learning monads in book learnyouahaskell. after reading writer monad, decided check documentation of control.monad.writer.class.

there saw implemented listen , pass functions not able understand used for. give me example can understand how use listen , pass?

here code control.monad.trans.writer.strict defined listen , pass:

listen :: (monoid w, monad m) => writert w m -> writert w m (a, w) listen m = writert $     (a, w) <- runwritert m     return ((a, w), w)  pass :: (monoid w, monad m) => writert w m (a, w -> w) -> writert w m pass m = writert $     ((a, f), w) <- runwritert m     return (a, f w) 

writert monad transformer provides writer functionality other monads, simple writer type defined type writer w = writert w identity, in identity monad bind function (<- , >>=) variable binding, if break down monadic part of code above:

listen :: (monoid w) => writer w -> writer w (a, w) listen m = writer $     let (a, w) = runwriter m     in ((a, w), w)  pass :: (monoid w) => writer w (a, w -> w) ->　writer w pass m = writer $     let ((a, f), w) = runwriter m     in (a, f w) 

it's clear listen gives access log produced writer action inside writer monad, , pass gives way alter log inside writer monad.

assume have writer [string] , want log action if log produced satisfies condition:

deleteon :: (monoid w) => (w -> bool) -> writer w -> writer w deleteon p m = pass $     (a, w) <- listen m     if p w         return (a, id)         else return (a, const mempty)  -- or pass alone deleteon' :: (monoid w) => (w -> bool) -> writer w -> writer w deleteon' p m = pass $     <- m     return (a, (\w -> if p w mempty else w))  logtwo :: writer [string] () logtwo =     deleteon ((> 5) . length . head) $ tell ["foo"]     deleteon ((> 5) . length . head) $ tell ["foobar"] {- *main> runwriter logtwo ((),["foo"]) -}