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.

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 necesarily flushed when the compiler finishes running, so you should flush them yourself.

reify looks up information about the Name.

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.

Obtained from reify in the Q Monad.

InstanceDec desribes 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 PrimTyConI, arity of the type constructor

In PrimTyConI, is the type constructor unlifted?

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.

reifyInstances nm tys returns a list of visible instances 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.

Is the list of instances returned by reifyInstances nonempty?

An abstract type representing names in the syntax tree.

Names can be constructed in several ways, which come with different name-capture guarantees (see Language.Haskell.TH.Syntax for an explanation of name capture):

• the built-in syntax 'f and ''T can be used to construct names, The expression 'f gives a Name which refers to the value f currently in scope, and ''T gives a Name which refers to the type T currently in scope. These names can never be captured.
• lookupValueName and lookupTypeName are similar to 'f and ''T respectively, but the Names are looked up at the point where the current splice is being run. These names can never be captured.
• newName monadically generates a new name, which can never be captured.
• mkName generates a capturable name.

Names constructed using newName and mkName may be used in bindings (such as let x = ... or x -> ...), but names constructed using lookupValueName, lookupTypeName, 'f, ''T may not.

Generate a capturable name. Occurrences of such names will be resolved according to the Haskell scoping rules at the occurrence site.

For example:

 f = [| pi + $(varE (mkName "pi")) |]
 ...
 g = let pi = 3 in $f

In this case, g is desugared to

 g = Prelude.pi + 3

Note that mkName may be used with qualified names:

 mkName "Prelude.pi"

See also dyn for a useful combinator. The above example could be rewritten using dyn as

 f = [| pi + $(dyn "pi") |]

Generate a fresh name, which cannot be captured.

For example, this:

f = $(do
  nm1 <- newName "x"
  let nm2 = mkName "x"
  return (LamE [VarP nm1] (LamE [VarP nm2] (VarE nm1)))
 )

will produce the splice

f = \x0 -> \x -> x0

In particular, the occurrence VarE nm1 refers to the binding VarP nm1, and is not captured by the binding VarP nm2.

Although names generated by newName cannot be captured, they can capture other names. For example, this:

g = $(do
  nm1 <- newName "x"
  let nm2 = mkName "x"
  return (LamE [VarP nm2] (LamE [VarP nm1] (VarE nm2)))
 )

will produce the splice

g = \x -> \x0 -> x0

since the occurrence VarE nm2 is captured by the innermost binding of x, namely VarP nm1.

The name without its module prefix

Module prefix of a name, if it exists

Tuple type constructor

Tuple data constructor

Unboxed tuple type constructor

Unboxed tuple data constructor

Default fixity: infixl 9

Highest allowed operator precedence for Fixity constructor (answer: 9)

Pattern in Haskell given in {}

To avoid duplication between kinds and types, they are defined to be the same. Naturally, you would never have a type be StarT and you would never have a kind be SigT, but many of the other constructors are shared. Note that the kind Bool is denoted with ConT, not PromotedT. Similarly, tuple kinds are made with TupleT, not PromotedTupleT.

Use with caseE

Use with funD

Dynamically binding a variable (unhygenic)

Single-arg lambda