:: Monad m => [m a] -> m [a] -package:hledger-lib package:hledger

This is a version of sequence based on difference lists. It is slightly faster but we mostly use it because it uses the heap instead of the stack. This has the advantage that Neil Mitchell’s trick of limiting the stack size to discover space leaks doesn’t show this as a false positive.

Evaluate each monadic action in the structure from left to right, and collect the results. For a version that ignores the results see sequence_.

Examples

Basic usage: The first two examples are instances where the input and and output of sequence are isomorphic.

>>> sequence $ Right [1,2,3,4]
[Right 1,Right 2,Right 3,Right 4]

>>> sequence $ [Right 1,Right 2,Right 3,Right 4]
Right [1,2,3,4]

The following examples demonstrate short circuit behavior for sequence.

>>> sequence $ Left [1,2,3,4]
Left [1,2,3,4]

>>> sequence $ [Left 0, Right 1,Right 2,Right 3,Right 4]
Left 0