Safe HaskellNone




The public face of Template Haskell

For other documentation, refer to: http://www.haskell.org/haskellwiki/Template_Haskell


The monad and its operations

data Q a Source

runQ :: Quasi m => Q a -> m aSource



:: Q a

recover with this one

-> Q a

failing action

-> Q a 

reify :: Name -> Q InfoSource

reify looks up information about the Name

location :: Q LocSource

location gives you the Location at which this computation is spliced.

runIO :: IO a -> Q aSource

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.

reifyInstances :: Name -> [Type] -> Q [Dec]Source

classInstances looks up instaces of a class


data Name Source

For global names (NameG) we need a totally unique name, so we must include the name-space of the thing

For unique-numbered things (NameU), we've got a unique reference anyway, so no need for name space

For dynamically bound thing (NameS) we probably want them to in a context-dependent way, so again we don't want the name space. For example:

 let v = mkName "T" in [| data $v = $v |]

Here we use the same Name for both type constructor and data constructor

NameL and NameG are bound *outside* the TH syntax tree either globally (NameG) or locally (NameL). Ex:

 f x = $(h [| (map, x) |])

The map will be a NameG, and x wil be a NameL

These Names should never appear in a binding position in a TH syntax tree

mkName :: String -> NameSource

The string can have a ., thus Foo.baz, giving a dynamically-bound qualified name, in which case we want to generate a NameQ

Parse the string to see if it has a . in it so we know whether to generate a qualified or unqualified name It's a bit tricky because we need to parse

 Foo.Baz.x   as    Qual Foo.Baz x

So we parse it from back to front

nameBase :: Name -> StringSource

Base, unqualified name.



:: Int 
-> Name

Type constructor



:: Int 
-> Name

Data constructor

The algebraic data types

The lowercase versions (syntax operators) of these constructors are preferred to these constructors, since they compose better with quotations ([| |]) and splices ($( ... ))

data Dec Source


FunD Name [Clause]
{ f p1 p2 = b where decs }
ValD Pat Body [Dec]
{ p = b where decs }
DataD Cxt Name [TyVarBndr] [Con] [Name]
{ data Cxt x => T x = A x | B (T x)
       deriving (Z,W)}
NewtypeD Cxt Name [TyVarBndr] Con [Name]
{ newtype Cxt x => T x = A (B x)
       deriving (Z,W)}
TySynD Name [TyVarBndr] Type
{ type T x = (x,x) }
ClassD Cxt Name [TyVarBndr] [FunDep] [Dec]
{ class Eq a => Ord a where ds }
InstanceD Cxt Type [Dec]
{ instance Show w => Show [w]
       where ds }
SigD Name Type
{ length :: [a] -> Int }
ForeignD Foreign 
PragmaD Pragma
{ {--} }
FamilyD FamFlavour Name [TyVarBndr] (Maybe Kind)
{ type family T a b c :: * }
DataInstD Cxt Name [Type] [Con] [Name]
{ data instance Cxt x => T [x] = A x 
                                | B (T x)
       deriving (Z,W)}
NewtypeInstD Cxt Name [Type] Con [Name]
{ newtype instance Cxt x => T [x] = A (B x)
       deriving (Z,W)}
TySynInstD Name [Type] Type
{ type instance T (Maybe x) = (x,x) }


data Exp Source

The CompE constructor represents a list comprehension, and takes a [Stmt]. The result expression of the comprehension is the *last* of these, and should be a NoBindS.

E.g. translation:

 [ f x | x <- xs ]
 CompE [BindS (VarP x) (VarE xs), NoBindS (AppE (VarE f) (VarE x))]


VarE Name
{ x }
ConE Name
data T1 = C1 t1 t2; p = {C1} e1 e2
LitE Lit
{ 5 or c}
AppE Exp Exp
{ f x }
InfixE (Maybe Exp) Exp (Maybe Exp)
{x + y} or {(x+)} or {(+ x)} or {(+)}

It's a bit gruesome to use an Exp as the operator, but how else can we distinguish constructors from non-constructors? Maybe there should be a var-or-con type? Or maybe we should leave it to the String itself?

UInfixE Exp Exp Exp
{x + y}

See Note [Unresolved infix] at Language.Haskell.TH.Syntax

ParensE Exp
{ (e) }

See Note [Unresolved infix] at Language.Haskell.TH.Syntax

LamE [Pat] Exp
{  p1 p2 -> e }
TupE [Exp]
{ (e1,e2) }
UnboxedTupE [Exp]
{ () }
CondE Exp Exp Exp
{ if e1 then e2 else e3 }
LetE [Dec] Exp
{ let x=e1;   y=e2 in e3 }
CaseE Exp [Match]
{ case e of m1; m2 }
DoE [Stmt]
{ do { p <- e1; e2 }  }
CompE [Stmt]
{ [ (x,y) | x <- xs, y <- ys ] }
ArithSeqE Range
{ [ 1 ,2 .. 10 ] }
ListE [Exp]
{ [1,2,3] }
SigE Exp Type
{ e :: t }
RecConE Name [FieldExp]
{ T { x = y, z = w } }
RecUpdE Exp [FieldExp]
{ (f x) { z = w } }


data Con Source


NormalC Name [StrictType]
C Int a
RecC Name [VarStrictType]
C { v :: Int, w :: a }
InfixC StrictType Name StrictType
Int :+ a
ForallC [TyVarBndr] Cxt Con
forall a. Eq a => C [a]


data Type Source


ForallT [TyVarBndr] Cxt Type
forall vars. ctxt -> type
VarT Name
ConT Name
TupleT Int
(,), (,,), etc.
UnboxedTupleT Int
(), (), etc.
AppT Type Type
T a b
SigT Type Kind
t :: k

data Kind Source


ArrowK Kind Kind
k1 -> k2

type CxtSource


 = [Pred]
(Eq a, Ord b)

data Pred Source


ClassP Name [Type]
Eq (Int, a)
EqualP Type Type
F a ~ Bool

data Match Source


Match Pat Body [Dec]
case e of { pat -> body where decs }

data Clause Source


Clause [Pat] Body [Dec]
f { p1 p2 = body where decs }

data Body Source


GuardedB [(Guard, Exp)]
f p { | e1 = e2 | e3 = e4 } where ds
NormalB Exp
f p { = e } where ds

data Guard Source


NormalG Exp 
PatG [Stmt] 

data Stmt Source


BindS Pat Exp 
LetS [Dec] 
NoBindS Exp 
ParS [[Stmt]] 

data Lit Source


CharL Char 
StringL String 
IntegerL Integer

Used for overloaded and non-overloaded literals. We don't have a good way to represent non-overloaded literals at the moment. Maybe that doesn't matter?

RationalL Rational 
IntPrimL Integer 
WordPrimL Integer 
FloatPrimL Rational 
DoublePrimL Rational 
StringPrimL String

A primitive C-style string, type Addr#


data Pat Source

Pattern in Haskell given in {}


LitP Lit
{ 5 or c }
VarP Name
{ x }
TupP [Pat]
{ (p1,p2) }
UnboxedTupP [Pat]
{ () }
ConP Name [Pat]
data T1 = C1 t1 t2; {C1 p1 p1} = e
InfixP Pat Name Pat
foo ({x :+ y}) = e
UInfixP Pat Name Pat
foo ({x :+ y}) = e

See Note [Unresolved infix] at Language.Haskell.TH.Syntax

ParensP Pat

See Note [Unresolved infix] at Language.Haskell.TH.Syntax

TildeP Pat
{ ~p }
BangP Pat
{ !p }
AsP Name Pat
{ x @ p }
{ _ }
RecP Name [FieldPat]
f (Pt { pointx = x }) = g x
ListP [Pat]
{ [1,2,3] }
SigP Pat Type
{ p :: t }
ViewP Exp Pat
{ e -> p }


data Info Source

Obtained from reify in the Q Monad.


ClassI Dec [InstanceDec]

A class is reified to its declaration and a list of its instances

ClassOpI Name Type Name Fixity 
TyConI Dec 
FamilyI Dec [InstanceDec] 
PrimTyConI Name Int Bool 
DataConI Name Type Name Fixity 
VarI Name Type (Maybe Dec) Fixity 
TyVarI Name Type 

Library functions


type DecsQ = Q [Dec]Source

Constructors lifted to Q



Pattern Guards

match :: PatQ -> BodyQ -> [DecQ] -> MatchQSource

Use with caseE

clause :: [PatQ] -> BodyQ -> [DecQ] -> ClauseQSource

Use with funD


dyn :: String -> Q ExpSource

Dynamically binding a variable (unhygenic)

lam1E :: PatQ -> ExpQ -> ExpQSource

Single-arg lambda


Ranges with more indirection




Class Contexts

Top Level Declarations


valD :: PatQ -> BodyQ -> [DecQ] -> DecQSource

dataD :: CxtQ -> Name -> [TyVarBndr] -> [ConQ] -> [Name] -> DecQSource


classD :: CxtQ -> Name -> [TyVarBndr] -> [FunDep] -> [DecQ] -> DecQSource

Type Family / Data Family

dataInstD :: CxtQ -> Name -> [TypeQ] -> [ConQ] -> [Name] -> DecQSource

Foreign Function Interface (FFI)


Just inline supported so far


pprint :: Ppr a => a -> StringSource