Portability | Haskell98 |
---|---|

Stability | stable |

Maintainer | wren@community.haskell.org |

Safe Haskell | Safe-Inferred |

This module provides safe zipping functions which will fail
(return `Nothing`

) on uneven length lists.

- pairWithBy :: (a -> b -> c) -> (c -> d -> d) -> d -> [a] -> [b] -> Maybe d
- pairWith :: (a -> b -> c) -> [a] -> [b] -> Maybe [c]
- pairBy :: ((a, b) -> d -> d) -> d -> [a] -> [b] -> Maybe d
- pair :: [a] -> [b] -> Maybe [(a, b)]
- biject :: [a -> b] -> [a] -> Maybe [b]
- biject' :: [a -> b] -> [a] -> Maybe [b]
- zipWithBy :: (a -> b -> c) -> (c -> d -> d) -> d -> [a] -> [b] -> d
- zipBy :: ((a, b) -> d -> d) -> d -> [a] -> [b] -> d

# Safe functions for zipping lists

pairWithBy :: (a -> b -> c) -> (c -> d -> d) -> d -> [a] -> [b] -> Maybe dSource

A generic version of `pair`

. The first argument is a tuple
homomorphism (i.e. a function for how to combine values from the
two lists), the second two arguments form a list homomorphism
(i.e. so you can `foldr`

the `[c]`

list directly without actually
constructing it).

In order to evaluate to WHNF `pairWithBy`

is strict in both list
arguments, as it must be, to determine that the lists are of the
same length. This means it can survive one infinite list (yielding
`Nothing`

) but that it can't survive two. The implementation is
very efficient and uses a tight tail-recursive loop, however
with extremely long lists it will be churning through heap and
that tightness can make it hard to interrupt (lists of 1 million
elements return in 1~2 seconds, but lists of 10 million can lock
your system up).

pairBy :: ((a, b) -> d -> d) -> d -> [a] -> [b] -> Maybe dSource

A safe version of `zip`

that uses a user-defined list homomorphism.

# Special safe zipping functions

biject :: [a -> b] -> [a] -> Maybe [b]Source

A bijection from a list of functions and a list of arguments to a list of results of applying the functions bijectively.

biject' :: [a -> b] -> [a] -> Maybe [b]Source

A version of `biject`

that applies functions strictly. N.B.
the list is still lazily evaluated, this just makes the functions
strict in their argument.

# New (unsafe) zipping functions

zipWithBy :: (a -> b -> c) -> (c -> d -> d) -> d -> [a] -> [b] -> dSource

An unsafe variant of `pairWithBy`

to fill out the interface.