Documentation

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])

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

normalizeTypeUnits ''Int    === [t| Int |]
normalizeTypeUnits ''Maybe  === [t| Maybe () |]
normalizeTypeUnits ''Either === [t| Either () () |]

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

The negation of isInstanceOf.

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 and newtype declarations and it is useful when generating typeclass instances.

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])]

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

Resolves a type synonym.

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

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

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

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

> showJustName ''Int
"Int"

> showJustName ''String
"String"

> showJustName ''Maybe
"Maybe"

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

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

Same as toBounded but lifted over Q