The Quote class implements the minimal interface which is necessary for desugaring quotations.

• The Monad m superclass is needed to stitch together the different AST fragments.
• newName is used when desugaring binding structures such as lambdas to generate fresh names.

Therefore the type of an untyped quotation in GHC is `Quote m => m Exp`

For many years the type of a quotation was fixed to be `Q Exp` but by more precisely specifying the minimal interface it enables the Exp to be extracted purely from the quotation without interacting with Q.

Use a Q computation in a Quote context. This function is only safe when the Q computation performs actions from the Quote instance for Q or any of Quote's subclasses (Functor, Applicative, and Monad). Attempting to perform actions from the MonadFail, MonadIO, or Quasi instances for Q will result in runtime errors.

This is useful when you have some Q-valued functions that only performs actions from Quote and wish to generalise it from Q to Quote without having to rewrite the internals of the function. This is especially handy for code defined in terms of combinators from Language.Haskell.TH.Lib, which were all hard-coded to Q prior to template-haskell-2.17.0.0. For instance, consider this function:

apply :: Exp -> Exp -> Q Exp
apply f x = appE (return x) (return y)

There are two ways to generalize this function to use Quote in a backwards-compatible way. One way to do so is to rewrite apply to avoid the use of appE, like so:

applyQuote :: Quote m => Exp -> Exp -> m Exp
applyQuote f x = return (AppE x y)

For a small example like applyQuote, there isn't much work involved. But this can become tiresome for larger examples. In such cases, unsafeQToQuote can do the heavy lifting for you. For example, applyQuote can also be defined as:

applyQuote :: Quote m => Exp -> Exp -> m Exp
applyQuote f x = unsafeQToQuote (apply f x)

Discard the type annotation and produce a plain Template Haskell expression

Levity-polymorphic since template-haskell-2.16.0.0.

This is a variant of the unTypeQ function that is always guaranteed to use a Quote constraint, even on old versions of template-haskell.

As this function interacts with typed Template Haskell, this function is only defined on template-haskell-2.9.0.0 (GHC 7.8) or later.

Annotate the Template Haskell expression with a type

This is unsafe because GHC cannot check for you that the expression really does have the type you claim it has.

Levity-polymorphic since template-haskell-2.16.0.0.

This is a variant of the unsafeTExpCoerce function that is always guaranteed to use a Quote constraint, even on old versions of template-haskell.

As this function interacts with typed Template Haskell, this function is only defined on template-haskell-2.9.0.0 (GHC 7.8) or later.

Turn a value into a Template Haskell expression, suitable for use in a splice.

This is a variant of the lift method of Lift that is always guaranteed to use a Quote constraint, even on old versions of template-haskell.

Levity-polymorphic since template-haskell-2.17.0.0.

Turn a value into a Template Haskell typed expression, suitable for use in a typed splice.

This is a variant of the liftTyped method of Lift that is always guaranteed to use a Quote constraint and return a Code, even on old versions of template-haskell.

As this function interacts with typed Template Haskell, this function is only defined on template-haskell-2.9.0.0 (GHC 7.8) or later. While the liftTyped method of Lift was first introduced in template-haskell-2.16.0.0, we are able to backport it back to template-haskell-2.9.0.0 by making use of the lift method on older versions of template-haskell. This crucially relies on the Lift law that lift x ≡ unTypeQ (liftTyped x) to work, so beware if you use liftTypedQuote with an unlawful Lift instance.

Levity-polymorphic since template-haskell-2.17.0.0.

This is a variant of the liftString function that is always guaranteed to use a Quote constraint, even on old versions of template-haskell.

Levity-polymorphic since template-haskell-2.16.0.0.

A class that allows one to smooth over the differences between Code m a (the type of typed Template Haskell quotations on template-haskell-2.17.0.0 or later) and m (TExp a) (the type of typed Template Haskell quotations on older versions of template-haskell). Here are two examples that demonstrate how to use each method of IsCode:

{-# LANGUAGE TemplateHaskell #-}

import Language.Haskell.TH
import Language.Haskell.TH.Syntax.Compat

-- toCode will ensure that the end result is a Code, regardless of
-- whether the quote itself returns a Code or a TExp.
myCode :: Code Q Int
myCode = toCode [|| 42 ||]

-- fromCode will ensure that the input Code is suitable for splicing
-- (i.e., it will return a Code or a TExp depending on the
-- template-haskell version in use).
fortyTwo :: Int
fortyTwo = $$(fromCode myCode)

Levity-polymorphic since template-haskell-2.16.0.0.

Unsafely convert an untyped code representation into a typed code representation.

Levity-polymorphic since template-haskell-2.16.0.0.

Lift a monadic action producing code into the typed Code representation

Levity-polymorphic since template-haskell-2.16.0.0.

Extract the untyped representation from the typed representation

Levity-polymorphic since template-haskell-2.16.0.0.

Modify the ambient monad used during code generation. For example, you can use hoistCode to handle a state effect:

 handleState :: Code (StateT Int Q) a -> Code Q a
 handleState = hoistCode (flip runState 0)

Levity-polymorphic since template-haskell-2.16.0.0.

Variant of (>>=) which allows effectful computations to be injected into code generation.

Levity-polymorphic since template-haskell-2.16.0.0.

Variant of (>>) which allows effectful computations to be injected into code generation.

Levity-polymorphic since template-haskell-2.16.0.0.

A useful combinator for embedding monadic actions into Code myCode :: ... => Code m a myCode = joinCode $ do x <- someSideEffect return (makeCodeWith x)

Levity-polymorphic since template-haskell-2.16.0.0.

Splice m a is a type alias for:

• Code m a, if using template-haskell-2.17.0.0 or later, or
• m (TExp a), if using an older version of template-haskell.

This should be used with caution, as its definition differs depending on which version of template-haskell you are using. It is mostly useful for contexts in which one is writing a definition that is intended to be used directly in a typed Template Haskell splice, as the types of TH splices differ between template-haskell versions as well.

Levity-polymorphic since template-haskell-2.16.0.0.

SpliceQ a is a type alias for:

• Code Q a, if using template-haskell-2.17.0.0 or later, or
• Q (TExp a), if using an older version of template-haskell.

This should be used with caution, as its definition differs depending on which version of template-haskell you are using. It is mostly useful for contexts in which one is writing a definition that is intended to be used directly in a typed Template Haskell splice, as the types of TH splices differ between template-haskell versions as well.

Levity-polymorphic since template-haskell-2.16.0.0.

A variant of bindCode that works over Splices. Because this function uses Splice, the type of this function will be different depending on which version of template-haskell you are using. (See the Haddocks for Splice for more information on this point.)

Levity-polymorphic since template-haskell-2.16.0.0.

A variant of bindCode_ that works over Splices. Because this function uses Splice, the type of this function will be different depending on which version of template-haskell you are using. (See the Haddocks for Splice for more information on this point.)

Levity-polymorphic since template-haskell-2.16.0.0.

A variant of examineCode that takes a Splice as an argument. Because this function takes a Splice as an argyment, the type of this function will be different depending on which version of template-haskell you are using. (See the Haddocks for Splice for more information on this point.)

Levity-polymorphic since template-haskell-2.16.0.0.

A variant of hoistCode that works over Splices. Because this function uses Splice, the type of this function will be different depending on which version of template-haskell you are using. (See the Haddocks for Splice for more information on this point.)

Levity-polymorphic since template-haskell-2.16.0.0.

A variant of joinCode that works over Splices. Because this function uses Splice, the type of this function will be different depending on which version of template-haskell you are using. (See the Haddocks for Splice for more information on this point.)

Levity-polymorphic since template-haskell-2.16.0.0.

A variant of liftCode that returns a Splice. Because this function returns a Splice, the return type of this function will be different depending on which version of template-haskell you are using. (See the Haddocks for Splice for more information on this point.)

Levity-polymorphic since template-haskell-2.16.0.0.

A variant of liftTypedQuote that is:

1. Always implemented in terms of lift behind the scenes, and
2. Returns a Splice. This means that the return type of this function will be different depending on which version of template-haskell you are using. (See the Haddocks for Splice for more information on this point.)

This is especially useful for minimizing CPP in one particular scenario: implementing liftTyped in hand-written Lift instances where the corresponding lift implementation cannot be derived. For instance, consider this example from the text library:

instance Lift Text where
  lift = appE (varE 'pack) . stringE . unpack
#if MIN_VERSION_template_haskell(2,17,0)
  liftTyped = unsafeCodeCoerce . lift
#elif MIN_VERSION_template_haskell(2,16,0)
  liftTyped = unsafeTExpCoerce . lift
#endif

The precise details of how this lift implementation works are not important, only that it is something that DeriveLift could not generate. The main point of this example is to illustrate how tiresome it is to write the CPP necessary to define liftTyped in a way that works across multiple versions of template-haskell. With liftTypedFromUntypedSplice, however, this becomes slightly easier to manage:

instance Lift Text where
  lift = appE (varE 'pack) . stringE . unpack
#if MIN_VERSION_template_haskell(2,16,0)
  liftTyped = liftTypedFromUntypedSplice
#endif

Note that due to the way this function is defined, this will only work for Lift instances t such that (t :: Type). If you wish to manually define liftTyped for a type with a different kind, you will have to use unsafeSpliceCoerce to overcome levity polymorphism restrictions.

Unsafely convert an untyped splice representation into a typed Splice representation. Because this function returns a Splice, the return type of this function will be different depending on which version of template-haskell you are using. (See the Haddocks for Splice for more information on this point.)

This is especially useful for minimizing CPP when:

1. You need to implement liftTyped in a hand-written Lift instance where the corresponding lift implementation cannot be derived, and
2. The data type receiving a Lift instance has a kind besides Type.

Condition (2) is important because while it is possible to simply define 'Syntax.liftTyped = liftTypedFromUntypedSplice for Lift instances t such that (t :: Type), this will not work for types with different types, such as unboxed types or unlifted newtypes. This is because GHC restrictions prevent defining liftTypedFromUntypedSplice in a levity polymorphic fashion, so one must use unsafeSpliceCoerce to work around these restrictions. Here is an example of how to use unsafeSpliceCoerce:

instance Lift Int# where
  lift x = litE (intPrimL (fromIntegral (I# x)))
#if MIN_VERSION_template_haskell(2,16,0)
  liftTyped x = unsafeSpliceCoerce (lift x)
#endif

Levity-polymorphic since template-haskell-2.16.0.0.

A variant of unTypeCode that takes a Splice as an argument. Because this function takes a Splice as an argyment, the type of this function will be different depending on which version of template-haskell you are using. (See the Haddocks for Splice for more information on this point.)

Levity-polymorphic since template-haskell-2.16.0.0.

Lift a TExp a into a Splice. This is useful when splicing in the result of a computation into a typed QuasiQuoter.

One example is traverseing over a list of elements and returning an expression from each element.

mkInt :: String -> Q (TExp Int)
mkInt str = [|| length $$str ||]

mkInts :: [String] -> Q [TExp Int]
mkInts = traverse mkInt

This gives us a list of TExp, not a TExp of a list. We can push the list inside the type with this function:

listTE :: [TExp a] -> TExp [a]
listTE = TExp . ListE . map unType

In a do block using liftSplice, we can bind the resulting

TExp [Int] out of the expression.

foo :: Q (TExp Int)
foo = do
     ints <- mkInts ["hello", "world", "goodybe", "bob"]
     [|| sum $$(pure (listTE ints)) ||]

Prior to GHC 9, with the Q type, we can write pure :: TExp a -> Q (TExp a), which is a valid thing to use in a typed quasiquoter. However, after GHC 9, this code will fail to type check. There is no Applicative instance for Code m a, so we need another way to splice it in.

A GHC 9 only solution can use Code :: m (TExp a) -> Code m a and pure together, like: Code . pure.

With expToSplice, we can splice it in a backwards compatible way. A fully backwards- and forwards-compatible example looks like this:

mkInt :: String -> Q Int
mkInt str = examineSplice [|| length $$str ||]

mkInts :: [String] -> Q [TExp Int]
mkInts = traverse mkInt

foo :: SpliceQ Int
foo = liftSplice $ do
     ints <- mkInts ["hello", "world", "goodybe", "bob"]
     examineSplice [|| sum $$(expToSplice (listTE ints)) ||]

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