case is:module

Case and case mapping related functions. This module provides full predicates and mappings that are not compatible with those in Data.Char, which rely on simple properties. See Unicode.Char.Case.Compat for a drop-in replacement of the functions in Data.Char.
Generic case analysis using generics-sop. "Case analysis" functions are those which take one function for each constructor of a sum type, examine a value of that type, and call the relevant function depending on which constructor was used to build that type. Examples include maybe, either and bool. It's often useful to define similar functions on user-defined sum types, which is boring at best and error-prone at worst. This module gives us these functions for any type which implements Generic. For any single-constructor types, such as tuples, this gives us generic uncurrying without any extra effort - see tupleL, tuple3L.

Example

Let's use These from these as an example. First we need an instance of Generic, which we can derive.
{-# LANGUAGE DeriveGeneric #-}
import qualified GHC.Generics as G
import Generics.SOP (Generic)

data These a b
= This a
| That b
| These a b
deriving (Show, Eq, G.Generic)

instance Generic (These a b)      -- we could also do this using DeriveAnyClass
We're going to re-implement the case analysis function these, using gcase. Our type has 3 constructors, so our function will have 4 arguments: one for the These we're analysing, and one function for each constructor. The function is polymorphic in the result type.
these ::
forall a b c.
These a b ->
_ -> _ -> _ ->
c
What are the types of those 3 functions? For each constructor, we make a function type taking one of each of the argument types, and returning our polymorphic result type c:
these ::
forall a b c.
These a b ->
(a -> c) ->       -- for This
(b -> c) ->       -- for That
(a -> b -> c) ->  -- for These
c
Finally, we add the implementation, which is just gcase:
these ::
forall a b c.
These a b ->
(a -> c) ->
(b -> c) ->
(a -> b -> c) ->
c
these = gcase
Note that we could have written the entire thing more succintly using Analysis:
these ::
forall a b c.
Analysis (These a b) c
these = gcase

Flipping the argument order

maybe, either and bool have a slightly different shape to these: they take the datatype (Maybe a, Either a b or Bool) after the case functions, whereas these (and generally any analysis function implemented using gcase) takes the datatype as its first argument, followed by the case functions. This is due to the implementation, and is the recommended usage due to performance. However, you may want your function to follow the same pattern as maybe, since this is more ergonomic. In this case you can use AnalysisR and gcaseR:
theseR ::
forall a b c.
(a -> c) ->
(b -> c) ->
(a -> b -> c) ->
These a b ->
c
-- alternate signature: theseR :: forall a b c. AnalysisR (These a b) c
theseR = gcaseR @(These a b)
Note that we need the TypeApplications extension here. If you're really against this extension, see gcaseR_.
This module is intended to be imported qualified. May I suggest:
import           Data.CaseInsensitive  ( CI )
import qualified Data.CaseInsensitive as CI
Note that the FoldCase instance for ByteStrings is only guaranteed to be correct for ISO-8859-1 encoded strings!
A couple of demonstrations for the caseSplit function.
Use TP to prove 2n^2 + n + 1 is never divisible by 3.
Provides functions that facilitate defining textcase transformations. To see how these can be used used, see the definitions of addTextCase in Citeproc.Pandoc and Citproc.CslJson.
Exposes subspecies types of Char. e.g. `[a-z]`, `[A-Z]`, and `[0-9]`.
Exposes naming cases.
An opaque structure representing a test case.