Safe Haskell	Safe-Inferred
Language	Haskell2010

Database.SQLFragment.TypeFamilies

Contents

List retypings

Description

Defines common type families

Synopsis

Documentation

type family UnMaybe a

Remove the Maybe of a Type

Equations

UnMaybe (Maybe a) = a
UnMaybe a = a

type family NotNull as

Remove Maybe on all types of a list

Equations

NotNull [] = []
NotNull (a : as) = UnMaybe a : NotNull as

type family Nullable as

Add Maybe on all types of a list if necessary

Equations

Nullable [] = []
Nullable (a : as) = Maybe (UnMaybe a) : Nullable as

type family IfMaybe a b

Wrap a type in a Maybe if the source type is a Maybe Usefull to *convert* types whilst preserving the maybenes. Example, in SQL the type of "$1>0" !%! "hello" is '[Bool] or '[Maybe Bool] if the value is nullable.

Equations

IfMaybe (Maybe a) b = Maybe b
IfMaybe a b = b

type family IfMaybe2 a b c

Wrap a destination type in a `Maybe if one of the source types is also a Maybe.

Equations

IfMaybe2 (Maybe a) b c = Maybe c
IfMaybe2 a (Maybe b) c = Maybe c
IfMaybe2 a b c = c

type family GetLabels as

Extract labels (proxy) of list of types

Equations

GetLabels [] = []
GetLabels (Tagged l a : as) = a : GetLabels as
GetLabels (a : as) = GetLabels as

type family GetLabel a :: k

Equations

GetLabel (Tagged (l :: k) a) = l
GetLabel a = ()

type family GetValues as

Equations

GetValues [] = []
GetValues (a : as) = GetValue a : GetValues as

type family GetValue a

Equations

GetValue (Tagged (l :: k) a) = a
GetValue a = a

class Boolable b

Instances

Boolable * Bool
Boolable * (Maybe Bool)
Boolable * b => Boolable * (Tagged k l b)

class Boolables bs

Instances

Boolables ([] *)
(Boolable * a, Boolables as) => Boolables ((:) * a as)

List retypings

type family RelabelList ls ts

Equations

RelabelList [] t = RelabelList [] t
RelabelList l [] = RelabelList l []
RelabelList (Tagged s b : ls) (t : ts) = Tagged s t : RelabelList ls ts