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Asked: 2026-05-28T15:25:38+00:00

Can someone please explain or give some resources on how function composition works in

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Can someone please explain or give some resources on how function composition works in relation to laziness?

For example how does filter (/='W') . map toUpper $ "justaword" work in Haskell compared to it’s counterpart in erlang which is not lazy?

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  1. Editorial Team

    Every time another character is demanded (or notification of end), the next character – if any – is mapped to uppercase, that is compared to ‘W’, delivered if unequal.

    filter (/= 'W') . map toUpper $ "justaword"
~> filter (/= 'W') (toUpper 'j' : map toUpper "ustaword")
~> filter (/= 'W') ('J' : map toUpper "ustaword")
~> 'J' : filter (/= 'W') (map toUpper "ustaword")

    Now the first character is available, so for queries like null or functions like take 1, no further work is done. If more characters are demanded by the consumer, they will be produced one by one until the end of the string is reached.

    Example:

    Prelude Data.Char> take 10 . filter (/= 'W') . map toUpper $ repeat 't'
"TTTTTTTTTT"

    repeat produces an infinite list, but as long as only a finite part is consumed, the computation finishes in finite time. However, take 10 . filter (/= 'W') . map toUpper $ repeat 'w' would not terminate, since none of the produced characters passes the filter to reach the take 10.

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