In short, Q provides the Quasi operations in one neat monad for the user.

The longer story, is that Q wraps an arbitrary Quasi-able monad. The perceptive reader notices that Quasi has only two instances, Q itself and IO, neither of which have concrete implementations.Q plays the trick of dependency inversion, providing an abstract interface for the user which is later concretely fufilled by an concrete Quasi instance, internal to GHC.

"Runs" the Q monad. Normal users of Template Haskell should not need this function, as the splice brackets $( ... ) are the usual way of running a Q computation.

This function is primarily used in GHC internals, and for debugging splices by running them in IO.

Note that many functions in Q, such as reify and other compiler queries, are not supported when running Q in IO; these operations simply fail at runtime. Indeed, the only operations guaranteed to succeed are newName, runIO, reportError and reportWarning.

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.

Report an error to the user, but allow the current splice's computation to carry on. To abort the computation, use fail.

Report a warning to the user, and carry on.

Report an error (True) or warning (False), but carry on; use fail to stop.

Recover from errors raised by reportError or fail.

The location at which this computation is spliced.

A location within a source file.

The runIO function lets you run an I/O computation in the Q monad. Take care: you are guaranteed the ordering of calls to runIO within a single Q computation, but not about the order in which splices are run.

Note: for various murky reasons, stdout and stderr handles are not necessarily flushed when the compiler finishes running, so you should flush them yourself.

reify looks up information about the Name. It will fail with a compile error if the Name is not visible. A Name is visible if it is imported or defined in a prior top-level declaration group. See the documentation for newDeclarationGroup for more details.

It is sometimes useful to construct the argument name using lookupTypeName or lookupValueName to ensure that we are reifying from the right namespace. For instance, in this context:

data D = D

which D does reify (mkName "D") return information about? (Answer: D-the-type, but don't rely on it.) To ensure we get information about D-the-value, use lookupValueName:

do
  Just nm <- lookupValueName "D"
  reify nm

and to get information about D-the-type, use lookupTypeName.

reifyModule mod looks up information about module mod. To look up the current module, call this function with the return value of thisModule.

Template Haskell is capable of reifying information about types and terms defined in previous declaration groups. Top-level declaration splices break up declaration groups.

For an example, consider this code block. We define a datatype X and then try to call reify on the datatype.

module Check where

data X = X
    deriving Eq

$(do
    info <- reify ''X
    runIO $ print info
 )

This code fails to compile, noting that X is not available for reification at the site of reify. We can fix this by creating a new declaration group using an empty top-level splice:

data X = X
    deriving Eq

$(pure [])

$(do
    info <- reify ''X
    runIO $ print info
 )

We provide newDeclarationGroup as a means of documenting this behavior and providing a name for the pattern.

Since top level splices infer the presence of the $( ... ) brackets, we can also write:

data X = X
    deriving Eq

newDeclarationGroup

$(do
    info <- reify ''X
    runIO $ print info
 )

Obtained from reify in the Q Monad.

Obtained from reifyModule in the Q Monad.

InstanceDec describes a single instance of a class or type function. It is just a Dec, but guaranteed to be one of the following:

• InstanceD (with empty [Dec])
• DataInstD or NewtypeInstD (with empty derived [Name])
• TySynInstD

In ClassOpI and DataConI, name of the parent class or type

In UnboxedSumE and UnboxedSumP, the number associated with a particular data constructor. SumAlts are one-indexed and should never exceed the value of its corresponding SumArity. For example:

• (#_|#) has SumAlt 1 (out of a total SumArity of 2)
• (#|_#) has SumAlt 2 (out of a total SumArity of 2)

In UnboxedSumE, UnboxedSumT, and UnboxedSumP, the total number of SumAlts. For example, (#|#) has a SumArity of 2.

In PrimTyConI, arity of the type constructor

In PrimTyConI, is the type constructor unlifted?

The language extensions known to GHC.

Note that there is an orphan Binary instance for this type supplied by the GHC.LanguageExtensions module provided by ghc-boot. We can't provide here as this would require adding transitive dependencies to the template-haskell package, which must have a minimal dependency set.

List all enabled language extensions.

Determine whether the given language extension is enabled in the Q monad.

Look up the given name in the (type namespace of the) current splice's scope. See Language.Haskell.TH.Syntax for more details.

Look up the given name in the (value namespace of the) current splice's scope. See Language.Haskell.TH.Syntax for more details.

reifyFixity nm attempts to find a fixity declaration for nm. For example, if the function foo has the fixity declaration infixr 7 foo, then reifyFixity 'foo would return Just (Fixity 7 InfixR). If the function bar does not have a fixity declaration, then reifyFixity 'bar returns Nothing, so you may assume bar has defaultFixity.

reifyType nm attempts to find the type or kind of nm. For example, reifyType 'not returns Bool -> Bool, and reifyType ''Bool returns Type. This works even if there's no explicit signature and the type or kind is inferred.

reifyInstances nm tys returns a list of all visible instances (see below for "visible") of nm tys. That is, if nm is the name of a type class, then all instances of this class at the types tys are returned. Alternatively, if nm is the name of a data family or type family, all instances of this family at the types tys are returned.

Note that this is a "shallow" test; the declarations returned merely have instance heads which unify with nm tys, they need not actually be satisfiable.

• reifyInstances ''Eq [ TupleT 2 `AppT` ConT ''A `AppT` ConT ''B ] contains the instance (Eq a, Eq b) => Eq (a, b) regardless of whether A and B themselves implement Eq
• reifyInstances ''Show [ VarT (mkName "a") ] produces every available instance of Show

There is one edge case: reifyInstances ''Typeable tys currently always produces an empty list (no matter what tys are given).

In principle, the *visible* instances are * all instances defined in a prior top-level declaration group (see docs on newDeclarationGroup), or * all instances defined in any module transitively imported by the module being compiled

However, actually searching all modules transitively below the one being compiled is unreasonably expensive, so reifyInstances will report only the instance for modules that GHC has had some cause to visit during this compilation. This is a shortcoming: reifyInstances might fail to report instances for a type that is otherwise unusued, or instances defined in a different component. You can work around this shortcoming by explicitly importing the modules whose instances you want to be visible. GHC issue #20529 has some discussion around this.