caseof

A simple way to query constructors, like cases but slightly more concise.

Aimed at sum types with many constructors:

data Wiggle = Woo Int Char | Wibble Int deriving Show

There is a case predicate:

> $(isCaseOf 'Woo) (Woo 5 'a')
True

There is a Maybe-based matcher:

> $(maybeCaseOf 'Woo) (Woo 1 'a')
Just (1,'a')

There is a "map" over a constructor:

> $(mapCaseOf 'Left) succ (Left 3)
Left 4

There is a combinator which calls your function with n arguments, or passes the whole value to an "else" clause.

> $(caseOf 'Woo) (\x y -> show x ++ show y) (const "") (Wibble 5)
""

In your stack.yaml, put:

extra-deps:
- git: https://github.com/chrisdone/caseof.git
  commit: 9a7f6bb

For me, the lens approach (aside from being a big dependency) has these downsides: a complicated set of types and a requirement to derive prisms for the types, which enter the namespace. The downside for caseof is the TH requirement, but it is dead simple. You just read this README in 20 seconds.