This package contains a family of functions based on familiar Applicative, Functor, Traversable, and Foldable methods but with the ability to operate on stacks of arbitrary depth.

For example, liftAll is like the liftA2 and liftA3 functions except that it can lift a function through multiple layers of Applicatives instead of just one. Additionally, it can lift a function of arbitrary arity, not just a 2 argument function like with liftA2.

The term "stack" is used in this library's documentation to refer to something like

Maybe [Either Bool [Int]]

where Int is embedded in a nesting of types that all share some common interface, such as Applicative, Traversable, or Foldable.

It's necessary to turn on TypeApplications so that you can supply a type argument that tells the compiler about the stack you are operating on. You specify the stack by giving it's type with () as the embedded value. The () should be at the level in the stack at which your target value is embedded. Here is an example GHCi session:

>>> :set -XTypeApplications
>>> liftAll @[Maybe [()]] (+) [Just [3 :: Int]] [Just [6, 7]]
[Just [9,10]]
>>> data Foo = Foo Int String Double deriving Show
>>> liftAll @[[()]] Foo [[2]] [["string"]] [[5]]
[[Foo 2 "string" 5.0]]
>>> fmapAll @[Maybe ()] length [Just "string"]
[Just 6]
>>> traverseAll @(Either String [()]) print (Right [1,2,3])
1
2
3
Right [(),(),()]

(Additional examples are in the haddocks.)

• Control
  • Control.Lift