ListT package:pipes

The list monad transformer, which extends a monad with non-determinism The type variables signify:

• m - The base monad
• a - The values that the computation yields throughout its execution

For basic construction and composition of ListT computations, much can be accomplished using common typeclass methods.

• return corresponds to yield, yielding a single value.
• (>>=) corresponds to for, calling the second computation once for each time the first computation yields.
• mempty neither yields any values nor produces any effects in the base monad.
• (<>) sequences two computations, yielding all the values of the first followed by all the values of the second.
• lift converts an action in the base monad into a ListT computation which performs the action and yields a single value.

ListT is a newtype wrapper for Producer. You will likely need to use Select and enumerate to convert back and forth between these two types to take advantage of all the Producer-related utilities that Pipes.Prelude has to offer.

• To lift a plain list into a ListT computation, first apply each to turn the list into a Producer. Then apply the Select constructor to convert from Producer to ListT.
• For other ways to construct ListT computations, see the “Producers” section in Pipes.Prelude to build Producers. These can then be converted to ListT using Select.
• To aggregate the values from a ListT computation (for example, to compute the sum of a ListT of numbers), first apply enumerate to obtain a Producer. Then see the “Folds” section in Pipes.Prelude to proceed.

Run a self-contained ListT computation