Category:Monads: Difference between revisions
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:Nests functions which return their output in an envelope that includes a log string. Nesting ('composing') such functions generates a concatenated log of a chain of function applications.
;the Maybe monad
:Nests partial functions which return their output in a wrapper that includes a boolean flag – indicating whether or not the input value was legal. Composition of these functions avoids the need for exception handling when an illegal value is encountered somewhere along the chain of function applications. The '''invalid''' flag is threaded up through the monad, allowing all further attempts at function application to be bypassed.
;the List monad
:Nests functions which output ranges of possible values, rather than single values. Composing these functions yields cartesian products, and a convenient encoding of set comprehensions.
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