Data.Express.Utils.TH

Template Haskell utilities.

Documentation

reallyDeriveCascading :: Name -> (Name -> DecsQ) -> Name -> DecsQ

deriveWhenNeeded :: Name -> (Name -> DecsQ) -> Name -> DecsQ

deriveWhenNeededOrWarn :: Name -> (Name -> DecsQ) -> Name -> DecsQ

typeConArgs :: Name -> Q [Name]

typeConArgsThat :: Name -> (Name -> Q Bool) -> Q [Name]

typeConCascadingArgsThat :: Name -> (Name -> Q Bool) -> Q [Name]

normalizeType :: Name -> Q (Type, [Type])

Normalizes a type by applying it to necessary type variables making it accept zero type parameters. The normalized type is paired with a list of necessary type variables.

> putStrLn $(stringE . show =<< normalizeType ''Int)
(ConT ''Int, [])

> putStrLn $(stringE . show =<< normalizeType ''Maybe)
(AppT (ConT ''Maybe) (VarT ''a),[VarT ''a])

> putStrLn $(stringE . show =<< normalizeType ''Either)
(AppT (AppT (ConT ''Either) (VarT ''a)) (VarT ''b),[VarT ''a,VarT ''b])

> putStrLn $(stringE . show =<< normalizeType ''[])
(AppT (ConT ''[]) (VarT a),[VarT a])

normalizeTypeUnits :: Name -> Q Type

Normalizes a type by applying it to units to make it star-kinded. (cf. normalizeType)

isInstanceOf :: Name -> Name -> Q Bool

Given a type name and a class name, returns whether the type is an instance of that class. The given type must be star-kinded ( * ) and the given class double-star-kinded ( * -> * .

> putStrLn $(stringE . show =<< ''Int `isInstanceOf` ''Num)
True

> putStrLn $(stringE . show =<< ''Int `isInstanceOf` ''Fractional)
False

isntInstanceOf :: Name -> Name -> Q Bool

The negation of isInstanceOf.

typeArity :: Name -> Q Int

Given a type name, return the number of arguments taken by that type. Examples in partially broken TH:

> putStrLn $(stringE . show =<< typeArity ''Int)
0

> putStrLn $(stringE . show =<< typeArity ''Maybe)
1

> putStrLn $(stringE . show =<< typeArity ''Either)
2

> putStrLn $(stringE . show =<< typeArity ''[])
1

> putStrLn $(stringE . show =<< typeArity ''(,))
2

> putStrLn $(stringE . show =<< typeArity ''(,,))
3

> putStrLn $(stringE . show =<< typeArity ''String)
0

This works for Data's and Newtype's and it is useful when generating typeclass instances.

typeConstructors :: Name -> Q [(Name, [Type])]

Given a type Name, returns a list of its type constructor Names paired with the type arguments they take. the type arguments they take.

> putStrLn $(stringE . show =<< typeConstructors ''Bool)
[ ('False, [])
, ('True, [])
]

> putStrLn $(stringE . show =<< typeConstructors ''[])
[ ('[], [])
, ('(:), [VarT ''a, AppT ListT (VarT ''a)])
]

> putStrLn $(stringE . show =<< typeConstructors ''(,))
[('(,), [VarT (mkName "a"), VarT (mkName "b")])]

> data Point  =  Pt Int Int
> putStrLn $(stringE . show =<< typeConstructors ''Point)
[('Pt,[ConT ''Int, ConT ''Int])]

isTypeSynonym :: Name -> Q Bool

Is the given Name a type synonym?

> putStrLn $(stringE . show =<< isTypeSynonym 'show)
False

> putStrLn $(stringE . show =<< isTypeSynonym ''Char)
False

> putStrLn $(stringE . show =<< isTypeSynonym ''String)
True

typeSynonymType :: Name -> Q Type

Resolves a type synonym.

> putStrLn $(stringE . show =<< typeSynonymType ''String)
AppT ListT (ConT ''Char)

mergeIFns :: DecsQ -> DecsQ

mergeI :: DecsQ -> DecsQ -> DecsQ

lookupValN :: String -> Q Name

Lookups the name of a value throwing an error when it is not found.

> putStrLn $(stringE . show =<< lookupValN "show")
'show

showJustName :: Name -> String

Encodes a Name as a String. This is useful when generating error messages.

> showJustName ''Int
"Int"

> showJustName ''String
"String"

> showJustName ''Maybe
"Maybe"

typeConstructorsArgNames :: Name -> Q [(Name, [Name])]

(|=>|) :: Cxt -> DecsQ -> DecsQ

(|++|) :: DecsQ -> DecsQ -> DecsQ

whereI :: DecsQ -> [Dec] -> DecsQ

unboundVars :: Type -> [Name]

Lists all unbound variables in a type. This intentionally excludes the ForallT constructor.

toBounded :: Type -> Type

Binds all unbound variables using a ForallT constructor. (cf. unboundVars)

toBoundedQ :: TypeQ -> TypeQ

Same as toBounded but lifted over Q

module Language.Haskell.TH