TH.Utilities

Miscellaneous Template Haskell utilities, added as needed by packages in the th-utilities repo and elsewhere.

Documentation

tyVarBndrName :: TyVarBndr_ flag -> Name

Get the Name of a TyVarBndr

appsT :: Type -> [Type] -> Type

unAppsT :: Type -> [Type]

Breaks a type application like A b c into [A, b, c]. In other words, it descends leftwards down AppT constructors, and yields a list of the results.

tupT :: [Q Type] -> Q Type

Given a list of types, produce the type of a tuple of those types. This is analogous to tupE and tupP.

tupT [[t|Int|], [t|Char|], [t|Bool]] = [t| (Int, Char, Bool) |]

@since FIXME

promotedTupT :: [Q Type] -> Q Type

Given a list of types, produce the type of a promoted tuple of those types. This is analogous to tupE and tupP.

promotedTupT [[t|3|], [t| 'True|], [t|Bool]] = [t| '(3, 'True, Bool) |]

@since FIXME

typeToNamedCon :: Type -> Maybe (Name, [Type])

Given a Type, returns a Just value if it's a named type constructor applied to arguments. This value contains the name of the type and a list of arguments.

expectTyCon1 :: Name -> Type -> Q Type

Expect the provided type to be an application of a regular type to one argument, otherwise fail with a message. This will also work if the name is a promoted data constructor (PromotedT).

expectTyCon2 :: Name -> Type -> Q (Type, Type)

Expect the provided type to be an application of a regular type to two arguments, otherwise fail with a message. This will also work if the name is a promoted data constructor (PromotedT).

proxyE :: TypeQ -> ExpQ

Given a type, construct the expression (Proxy :: Proxy ty).

everywhereButStrings :: Data a => (forall b. Data b => b -> b) -> a -> a

Like the everywhere generic traversal strategy, but skips over strings. This can aid performance of TH traversals quite a bit.

everywhereButStringsM :: forall a m. (Data a, Monad m) => GenericM m -> a -> m a

Like the everywhereM generic traversal strategy, but skips over strings. This can aid performance of TH traversals quite a bit.

toSimpleName :: Name -> Name

Make a Name with a NameS or NameQ flavour, from a Name with any NameFlavour. This may change the meaning of names.

dequalify :: Name -> Name

Construct a plain name (mkName) based on the given name. This is useful for cases where TH doesn't expect a unique name.

dequalifyTyVars :: Data a => a -> a

Apply dequalify to every type variable.

freeVarsT :: Type -> [Name]

Get the free type variables of a Type.

plainInstanceD :: Cxt -> Type -> [Dec] -> Dec

Utility to conveniently handle change to InstanceD API in template-haskell-2.11.0

fromPlainInstanceD :: Dec -> Maybe (Cxt, Type, [Dec])

Utility to conveniently handle change to InstanceD API in template-haskell-2.11.0

typeRepToType :: TypeRep -> Q Type

Utility to convert Data.Typeable TypeRep to a Type. Note that this function is known to not yet work for many cases, but it does work for normal user datatypes. In future versions this function might have better behavior.

data ExpLifter

Hack to enable putting expressions inside lift-ed TH data. For example, you could do

    main = print $(lift [ExpLifter [e| 1 + 1 |],  ExpLifter [e| 2 |]])

Here, lift is working on a value of type [ExpLifter]. The code generated by lift constructs a list with the ExpLifter expressions providing the element values.

Without ExpLifter, lift tends to just generate code involving data construction. With ExpLifter, you can put more complicated expression into this construction.

Note that this cannot be used in typed quotes, because liftTyped will throw an exception. This is because this hack is incompatible with the type of liftTyped, as it would require the generated code to have type ExpLifter.

Constructors

ExpLifter (forall m. Quote m => m Exp)

Instances

Lift ExpLifter

Defined in TH.Utilities

Methods

lift :: Quote m => ExpLifter -> m Exp

liftTyped :: forall (m :: Type -> Type). Quote m => ExpLifter -> Code m ExpLifter

dumpSplices :: DecsQ -> DecsQ

Print splices generated by a TH splice (the printing will happen during compilation, as a GHC warning). Useful for debugging.

For instance, you can dump splices generated with makeLenses by replacing a top-level invocation of makeLenses in your code with:

dumpSplices $ makeLenses ''Foo