Language.Haskell.TH.Syntax

class Monad m => Quasi m where Source

badIO :: String -> IO a Source

counter :: IORef Int Source

newtype Q a Source

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

newtype TExp a Source

unTypeQ :: Q (TExp a) -> Q Exp Source

unsafeTExpCoerce :: Q Exp -> Q (TExp a) Source

newName :: String -> Q Name Source

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.

report :: Bool -> String -> Q () Source

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

reportError :: String -> Q () Source

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

reportWarning :: String -> Q () Source

Report a warning to the user, and carry on.

recover Source

Recover from errors raised by reportError or fail.

lookupName :: Bool -> String -> Q (Maybe Name) Source

lookupTypeName :: String -> Q (Maybe Name) Source

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

lookupValueName :: String -> Q (Maybe Name) Source

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

The functions lookupTypeName and lookupValueName provide a way to query the current splice's context for what names are in scope. The function lookupTypeName queries the type namespace, whereas lookupValueName queries the value namespace, but the functions are otherwise identical.

A call lookupValueName s will check if there is a value with name s in scope at the current splice's location. If there is, the Name of this value is returned; if not, then Nothing is returned.

The returned name cannot be "captured". For example:

f = "global"
g = $( do
         Just nm <- lookupValueName "f"
         [| let f = "local" in $( varE nm ) |]

In this case, g = "global"; the call to lookupValueName returned the global f, and this name was not captured by the local definition of f.

The lookup is performed in the context of the top-level splice being run. For example:

f = "global"
g = $( [| let f = "local" in
           $(do
               Just nm <- lookupValueName "f"
               varE nm
            ) |] )

Again in this example, g = "global", because the call to lookupValueName queries the context of the outer-most $(...).

Operators should be queried without any surrounding parentheses, like so:

lookupValueName "+"

Qualified names are also supported, like so:

lookupValueName "Prelude.+"
lookupValueName "Prelude.map"

reify :: Name -> Q Info Source

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.

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

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.

reifyRoles :: Name -> Q [Role] Source

reifyRoles nm returns the list of roles associated with the parameters of the tycon nm. Fails if nm cannot be found or is not a tycon. The returned list should never contain InferR.

reifyAnnotations :: Data a => AnnLookup -> Q [a] Source

reifyAnnotations target returns the list of annotations associated with target. Only the annotations that are appropriately typed is returned. So if you have Int and String annotations for the same target, you have to call this function twice.

reifyModule :: Module -> Q ModuleInfo Source

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

isInstance :: Name -> [Type] -> Q Bool Source

Is the list of instances returned by reifyInstances nonempty?

location :: Q Loc Source

The location at which this computation is spliced.

runIO :: IO a -> Q a Source

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.

addDependentFile :: FilePath -> Q () Source

Record external files that runIO is using (dependent upon). The compiler can then recognize that it should re-compile the Haskell file when an external file changes.

Expects an absolute file path.

Notes:

• ghc -M does not know about these dependencies - it does not execute TH.
• The dependency is based on file content, not a modification time

addTopDecls :: [Dec] -> Q () Source

Add additional top-level declarations. The added declarations will be type checked along with the current declaration group.

addModFinalizer :: Q () -> Q () Source

Add a finalizer that will run in the Q monad after the current module has been type checked. This only makes sense when run within a top-level splice.

getQ :: Typeable a => Q (Maybe a) Source

Get state from the Q monad.

putQ :: Typeable a => a -> Q () Source

Replace the state in the Q monad.

returnQ :: a -> Q a Source

bindQ :: Q a -> (a -> Q b) -> Q b Source

sequenceQ :: [Q a] -> Q [a] Source

class Lift t where Source

liftString :: String -> Q Exp Source

trueName :: Name Source

falseName :: Name Source

nothingName :: Name Source

justName :: Name Source

leftName :: Name Source

rightName :: Name Source

newtype ModName Source

newtype PkgName Source

data Module Source

Obtained from reifyModule and thisModule.

newtype OccName Source

mkModName :: String -> ModName Source

modString :: ModName -> String Source

mkPkgName :: String -> PkgName Source

pkgString :: PkgName -> String Source

mkOccName :: String -> OccName Source

occString :: OccName -> String Source

Much of Name API is concerned with the problem of name capture, which can be seen in the following example.

f expr = [| let x = 0 in $expr |]
...
g x = $( f [| x |] )
h y = $( f [| y |] )

A naive desugaring of this would yield:

g x = let x = 0 in x
h y = let x = 0 in y

All of a sudden, g and h have different meanings! In this case, we say that the x in the RHS of g has been captured by the binding of x in f.

What we actually want is for the x in f to be distinct from the x in g, so we get the following desugaring:

g x = let x' = 0 in x
h y = let x' = 0 in y

which avoids name capture as desired.

In the general case, we say that a Name can be captured if the thing it refers to can be changed by adding new declarations.

data Name Source

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.

data NameFlavour Source

data NameSpace Source

type Uniq = Int Source

nameBase :: Name -> String Source

The name without its module prefix

nameModule :: Name -> Maybe String Source

Module prefix of a name, if it exists

mkName :: String -> Name Source

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") |]

mkNameU :: String -> Uniq -> Name Source

Only used internally

mkNameL :: String -> Uniq -> Name Source

Only used internally

mkNameG :: NameSpace -> String -> String -> String -> Name Source

Used for 'x etc, but not available to the programmer

mkNameG_v :: String -> String -> String -> Name Source

mkNameG_tc :: String -> String -> String -> Name Source

mkNameG_d :: String -> String -> String -> Name Source

data NameIs Source

showName :: Name -> String Source

showName' :: NameIs -> Name -> String Source

tupleDataName :: Int -> Name Source

Tuple data constructor

tupleTypeName :: Int -> Name Source

Tuple type constructor

mk_tup_name :: Int -> NameSpace -> Name Source

unboxedTupleDataName :: Int -> Name Source

Unboxed tuple data constructor

unboxedTupleTypeName :: Int -> Name Source

Unboxed tuple type constructor

mk_unboxed_tup_name :: Int -> NameSpace -> Name Source

data Loc Source

type CharPos Source

data Info Source

Obtained from reify in the Q Monad.

data ModuleInfo Source

Obtained from reifyModule in the Q Monad.

type ParentName = Name Source

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

type Arity = Int Source

In PrimTyConI, arity of the type constructor

type Unlifted = Bool Source

In PrimTyConI, is the type constructor unlifted?

type InstanceDec = Dec Source

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

data Fixity Source

data FixityDirection Source

maxPrecedence :: Int Source

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

defaultFixity :: Fixity Source

Default fixity: infixl 9

When implementing antiquotation for quasiquoters, one often wants to parse strings into expressions:

parse :: String -> Maybe Exp

But how should we parse a + b * c? If we don't know the fixities of + and *, we don't know whether to parse it as a + (b * c) or (a + b) * c.

In cases like this, use UInfixE or UInfixP, which stand for "unresolved infix expression" and "unresolved infix pattern". When the compiler is given a splice containing a tree of UInfixE applications such as

UInfixE
  (UInfixE e1 op1 e2)
  op2
  (UInfixE e3 op3 e4)

it will look up and the fixities of the relevant operators and reassociate the tree as necessary.

• trees will not be reassociated across ParensE or ParensP, which are of use for parsing expressions like

(a + b * c) + d * e

• InfixE and InfixP expressions are never reassociated.
• The UInfixE constructor doesn't support sections. Sections such as (a *) have no ambiguity, so InfixE suffices. For longer sections such as (a + b * c -), use an InfixE constructor for the outer-most section, and use UInfixE constructors for all other operators:

InfixE
  Just (UInfixE ...a + b * c...)
  op
  Nothing

Sections such as (a + b +) and ((a + b) +) should be rendered into Exps differently:

(+ a + b)   ---> InfixE Nothing + (Just $ UInfixE a + b)
                   -- will result in a fixity error if (+) is left-infix
(+ (a + b)) ---> InfixE Nothing + (Just $ ParensE $ UInfixE a + b)
                   -- no fixity errors

• Quoted expressions such as

[| a * b + c |] :: Q Exp
[p| a : b : c |] :: Q Pat

will never contain UInfixE, UInfixP, ParensE, or ParensP constructors.

data Lit Source

data Pat Source

Pattern in Haskell given in {}

{ 5 or c }

{ x }

{ (p1,p2) }

{ () }

data T1 = C1 t1 t2; {C1 p1 p1} = e

foo ({x :+ y}) = e

foo ({x :+ y}) = e

{(p)}

{ ~p }

{ !p }

{ x @ p }

{ _ }

f (Pt { pointx = x }) = g x

{ [1,2,3] }

{ p :: t }

{ e -> p }

type FieldPat = (Name, Pat) Source

data Match Source

case e of { pat -> body where decs }

data Clause Source

f { p1 p2 = body where decs }

data Exp Source

{ x }

data T1 = C1 t1 t2; p = {C1} e1 e2

{ 5 or c}

{ f x }

{x + y} or {(x+)} or {(+ x)} or {(+)}

{x + y}

{ (e) }

{  p1 p2 -> e }

{ case m1; m2 }

{ (e1,e2) }

{ () }

{ if e1 then e2 else e3 }

{ if | g1 -> e1 | g2 -> e2 }

{ let x=e1;   y=e2 in e3 }

{ case e of m1; m2 }

{ do { p <- e1; e2 }  }

{ [ (x,y) | x <- xs, y <- ys ] }

[ f x | x <- xs ]

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

{ [ 1 ,2 .. 10 ] }

{ [1,2,3] }

{ e :: t }

{ T { x = y, z = w } }

{ (f x) { z = w } }

{ static e }

type FieldExp = (Name, Exp) Source

data Body Source

f p { | e1 = e2
      | e3 = e4 }
 where ds

f p { = e } where ds

data Guard Source

f x { | odd x } = x

f x { | Just y <- x, Just z <- y } = z

data Stmt Source

data Range Source

data Dec Source

{ f p1 p2 = b where decs }

{ p = b where decs }

{ data Cxt x => T x = A x | B (T x)
       deriving (Z,W)}

{ newtype Cxt x => T x = A (B x)
       deriving (Z,W)}

{ type T x = (x,x) }

{ class Eq a => Ord a where ds }

{ instance Show w => Show [w]
       where ds }

{ length :: [a] -> Int }

{ foreign import ... }
{ foreign export ... }

{ infix 3 foo }

{ {--} }

{ type family T a b c :: * }

{ data instance Cxt x => T [x] = A x
                                | B (T x)
       deriving (Z,W)}

{ newtype instance Cxt x => T [x] = A (B x)
       deriving (Z,W)}

{ type instance ... }

{ type family F a b :: * where ... }

{ type role T nominal representational }

{ deriving instance Ord a => Ord (Foo a) }

{ default size :: Data a => a -> Int }

data TySynEqn Source

One equation of a type family instance or closed type family. The arguments are the left-hand-side type patterns and the right-hand-side result.

data FunDep Source

data FamFlavour Source

data Foreign Source

data Callconv Source

data Safety Source

data Pragma Source

data Inline Source

data RuleMatch Source

data Phases Source

data RuleBndr Source

data AnnTarget Source

type Cxt Source

(Eq a, Ord b)

type Pred = Type Source

Since the advent of ConstraintKinds, constraints are really just types. Equality constraints use the EqualityT constructor. Constraints may also be tuples of other constraints.

data Strict Source

data Con Source

C Int a

C { v :: Int, w :: a }

Int :+ a

forall a. Eq a => C [a]

type StrictType = (Strict, Type) Source

type VarStrictType = (Name, Strict, Type) Source

data Type Source

forall <vars>. <ctxt> -> <type>

T a b

t :: k

a

T

'T

(,), (,,), etc.

(), (), etc.

->

~

[]

'(), '(,), '(,,), etc.

'[]

(':)

*

Constraint

0,1,2, etc.

data TyVarBndr Source

a

(a :: k)

data TyLit Source

2

Hello

data Role Source

Role annotations

nominal

representational

phantom

_

data AnnLookup Source

Annotation target for reifyAnnotations

type Kind = Type Source

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.

cmpEq :: Ordering -> Bool Source

thenCmp :: Ordering -> Ordering -> Ordering Source