Wrap an existing transducer such that input and output of the transducer will be logged with Debug.log.

Because Elm 0.19 no longer allows published packages to use Debug.log, you must provide Debug.log as a parameter to this function. Also, because Elm lacks Rank-N types, you must provide Debug.log three times so it can be specialized for the input, the output, and the state of the transducer.

import Debug


filter' pred =
    debug Debug.log Debug.log Debug.log "filter" (filter pred)


main =
    show (transduceList (filter' ((/=) 2)) [ 1, 2, 3 ])




-- Console output:
--     filter: input: 3
--     filter: state: ()
--     filter -> : 3
--     filter: input: 2
--     filter: state: ()
--     filter: input: 1
--     filter: state: ()
--     filter -> : 1
--     filter: complete: ()