6.13. Template Haskell

Template Haskell allows you to do compile-time meta-programming in Haskell. The background to the main technical innovations is discussed in “Template Meta-programming for Haskell” (Proc Haskell Workshop 2002).

The Template Haskell page on the GHC Wiki has a wealth of information. You may also consult the Haddock reference documentation Language.Haskell.TH. Many changes to the original design are described in Notes on Template Haskell version 2. Not all of these changes are in GHC, however.

The first example from that paper is set out below (A Template Haskell Worked Example) as a worked example to help get you started.

The documentation here describes the realisation of Template Haskell in GHC. It is not detailed enough to understand Template Haskell; see the Wiki page.

6.13.1. Syntax

TemplateHaskell
Implies:TemplateHaskellQuotes
Since:6.0. Typed splices introduced in GHC 7.8.1.

Enable Template Haskell’s splice and quotation syntax.

TemplateHaskellQuotes
Since:8.0.1

Enable only Template Haskell’s quotation syntax.

Template Haskell has the following new syntactic constructions. You need to use the extension TemplateHaskell to switch these syntactic extensions on. Alternatively, the TemplateHaskellQuotes extension can be used to enable the quotation subset of Template Haskell (i.e. without top-level splices). The TemplateHaskellQuotes extension is considered safe under Safe Haskell while TemplateHaskell is not.

  • A splice is written $x, where x is an arbitrary expression. There must be no space between the “$” and the expression. This use of “$” overrides its meaning as an infix operator, just as “M.x” overrides the meaning of “.” as an infix operator. If you want the infix operator, put spaces around it.

    A top-level splice can occur in place of

    • an expression; the spliced expression must have type Q Exp
    • a pattern; the spliced pattern must have type Q Pat
    • a type; the spliced expression must have type Q Type
    • a list of declarations at top level; the spliced expression must have type Q [Dec]

    Inside a splice you can only call functions defined in imported modules, not functions defined elsewhere in the same module. Note that declaration splices are not allowed anywhere except at top level (outside any other declarations).

    The Q monad is a monad defined in Language.Haskell.TH.Syntax which supports several useful operations during code generation such as reporting errors or looking up identifiers in the environment.

  • A expression quotation is written in Oxford brackets, thus:

    • [| ... |], or [e| ... |], where the “…” is an expression; the quotation has type Quote m => m Exp.
    • [d| ... |], where the “…” is a list of top-level declarations; the quotation has type Quote m => m [Dec].
    • [t| ... |], where the “…” is a type; the quotation has type Quote m => m Type.
    • [p| ... |], where the “…” is a pattern; the quotation has type Quote m => m Pat.

    The Quote type class (Language.Haskell.TH.Syntax.Quote) is the minimal interface necessary to implement the desugaring of quotations. The Q monad is an instance of Quote but contains many more operations which are not needed for defining quotations.

    See Where can they occur? for using partial type signatures in quotations.

  • Splices can be nested inside quotation brackets. For example the fragment representing 1 + 2 can be constructed using nested splices:

    oneC, twoC, plusC  :: Quote m => m Exp
oneC = [| 1 |]

twoC = [| 2 |]

plusC = [| $oneC + $twoC |]

  • The precise type of a quotation depends on the types of the nested splices inside it:

    -- Add a redundant constraint to demonstrate that constraints on the
-- monad used to build the representation are propagated when using nested
-- splices.
f :: (Quote m, C m) => m Exp
f = [| 5 | ]

-- f is used in a nested splice so the constraint on f, namely C, is propagated
-- to a constraint on the whole representation.
g :: (Quote m, C m) => m Exp
g = [| $f + $f |]

    Remember, a top-level splice still requires its argument to be of type Q Exp. So then splicing in g will cause m to be instantiated to Q:

    h :: Int
h = $(g) -- m ~ Q

  • A typed expression splice is written $$x, where x is is an arbitrary expression.

    A top-level typed expression splice can occur in place of an expression; the spliced expression must have type Code Q a

    NOTE: Currently typed splices may inhibit the unused identifier warning for identifiers in scope. See %s16524.

  • A typed expression quotation is written as [|| ... ||], or [e|| ... ||], where the “…” is an expression; if the “…” expression has type a, then the quotation has type Quote m => Code m a.

    It is possible to extract a value of type m Exp from Code m a using the unTypeCode :: Code m a -> m Exp function.

  • A quasi-quotation can appear in a pattern, type, expression, or declaration context and is also written in Oxford brackets:

  • A name can be quoted with either one or two prefix single quotes:

    • 'f has type Name, and names the function f. Similarly 'C has type Name and names the data constructor C. In general '⟨thing⟩ interprets ⟨thing⟩ in an expression context.

      A name whose second character is a single quote cannot be quoted in exactly this way, because it will be parsed instead as a quoted character. For example, if the function is called f'7 (which is a legal Haskell identifier), an attempt to quote it as 'f'7 would be parsed as the character literal 'f' followed by the numeric literal 7. As for promoted constructors (Distinguishing between types and constructors), the workaround is to add a space between the quote and the name. The name of the function f'7 is thus written ' f'7.

    • ''T has type Name, and names the type constructor T. That is, ''⟨thing⟩ interprets ⟨thing⟩ in a type context.

    These Names can be used to construct Template Haskell expressions, patterns, declarations etc. They may also be given as an argument to the reify function.

  • It is possible for a splice to expand to an expression that contain names which are not in scope at the site of the splice. As an example, consider the following code:

    module Bar where

import Language.Haskell.TH

add1 :: Quote m => Int -> m Exp
add1 x = [| x + 1 |]

    Now consider a splice using add1 in a separate module:

    module Foo where

import Bar

two :: Int
two = $(add1 1)

    Template Haskell cannot know what the argument to add1 will be at the function’s definition site, so a lifting mechanism is used to promote x into a value of type Quote m => m Exp. This functionality is exposed to the user as the Lift typeclass in the Language.Haskell.TH.Syntax module. If a type has a Lift instance, then any of its values can be lifted to a Template Haskell expression:

    class Lift t where
    lift :: Quote m => t -> m Exp
    liftTyped :: Quote m => t -> Code m t

    In general, if GHC sees an expression within Oxford brackets (e.g., [| foo bar |], then GHC looks up each name within the brackets. If a name is global (e.g., suppose foo comes from an import or a top-level declaration), then the fully qualified name is used directly in the quotation. If the name is local (e.g., suppose bar is bound locally in the function definition mkFoo bar = [| foo bar |]), then GHC uses lift on it (so GHC pretends [| foo bar |] actually contains [| foo $(lift bar) |]). Local names, which are not in scope at splice locations, are actually evaluated when the quotation is processed.

    The template-haskell library provides Lift instances for many common data types. Furthermore, it is possible to derive Lift instances automatically by using the DeriveLift language extension. See Deriving Lift instances for more information.

  • You may omit the $(...) in a top-level declaration splice. Simply writing an expression (rather than a declaration) implies a splice. For example, you can write

    module Foo where
import Bar

f x = x

$(deriveStuff 'f)   -- Uses the $(...) notation

g y = y+1

deriveStuff 'g      -- Omits the $(...)

h z = z-1

    This abbreviation makes top-level declaration slices quieter and less intimidating.

  • Pattern splices introduce variable binders but scoping of variables in expressions inside the pattern’s scope is only checked when a splice is run. Note that pattern splices that occur outside of any quotation brackets are run at compile time. Pattern splices occurring inside a quotation bracket are not run at compile time; they are run when the bracket is spliced in, sometime later. For example,

    mkPat :: Quote m => m Pat
mkPat = [p| (x, y) |]

-- in another module:
foo :: (Char, String) -> String
foo $(mkPat) = x : z

bar :: Quote m => m Exp
bar = [| \ $(mkPat) -> x : w |]

    will fail with z being out of scope in the definition of foo but it will not fail with w being out of scope in the definition of bar. That will only happen when bar is spliced.

  • A pattern quasiquoter may generate binders that scope over the right-hand side of a definition because these binders are in scope lexically. For example, given a quasiquoter haskell that parses Haskell, in the following code, the y in the right-hand side of f refers to the y bound by the haskell pattern quasiquoter, not the top-level y = 7.

    y :: Int
y = 7

f :: Int -> Int -> Int
f n = \ [haskell|y|] -> y+n

  • Top-level declaration splices break up a source file into declaration groups. A declaration group is the group of declarations created by a top-level declaration splice, plus those following it, down to but not including the next top-level declaration splice. N.B. only top-level splices delimit declaration groups, not expression splices. The first declaration group in a module includes all top-level definitions down to but not including the first top-level declaration splice.

    Each group is compiled just like a separately compiled module. That is:

    • Later groups can “see” declarations, and instance declarations, from earlier groups;
    • But earlier groups cannot “see” declarations, or instance declarations, from later groups.

    Each declaration group is mutually recursive only within the group. Declaration groups can refer to definitions within previous groups, but not later ones.

    Accordingly, the type environment seen by reify includes all the top-level declarations up to the end of the immediately preceding declaration group, but no more.

    Unlike normal declaration splices, declaration quasiquoters do not cause a break. These quasiquoters are expanded before the rest of the declaration group is processed, and the declarations they generate are merged into the surrounding declaration group. Consequently, the type environment seen by reify from a declaration quasiquoter will not include anything from the quasiquoter’s declaration group.

    Concretely, consider the following code

    module M where

import ...

f x = x

$(th1 4)

h y = k y y $(blah1)

[qq|blah|]

k x y z = x + y + z

$(th2 10)

w z = $(blah2)

    In this example, a reify inside…

    1. The splice $(th1 ...) would see the definition of f - the splice is top-level and thus all definitions in the previous declaration group are visible (that is, all definitions in the module up-to, but not including, the splice itself).
    2. The splice $(blah1) cannot refer to the function w - w is part of a later declaration group, and thus invisible, similarly, $(blah1) cannot see the definition of h (since it is part of the same declaration group as $(blah1). However, the splice $(blah1) can see the definition of f (since it is in the immediately preceding declaration group).
    3. The splice $(th2 ...) would see the definition of f, all the bindings created by $(th1 ...), the definition of h and all bindings created by [qq|blah|] (they are all in previous declaration groups).
    4. The body of h can refer to the function k appearing on the other side of the declaration quasiquoter, as quasiquoters do not cause a declaration group to be broken up.
    5. The qq quasiquoter would be able to see the definition of f from the preceding declaration group, but not the definitions of h or k, or any definitions from subsequent declaration groups.
    6. The splice $(blah2) would see the same definitions as the splice $(th2 ...) (but not any bindings it creates).

    Note that since an expression splice is unable to refer to declarations in the same declaration group, we can introduce a top-level (empty) splice to break up the declaration group

    module M where

data D = C1 | C2

f1 = $(th1 ...)

$(return [])

f2 = $(th2 ...)

    Here

    1. The splice $(th1 ...) cannot refer to D - it is in the same declaration group.
    2. The declaration group containing D is terminated by the empty top-level declaration splice $(return []) (recall, Q is a Monad, so we may simply return the empty list of declarations).
    3. Since the declaration group containing D is in the previous declaration group, the splice $(th2 ...) can refer to D.

    Note that in some cases, the presence or absence of top-level declaration splices can affect the runtime behavior of the surrounding code, because the resolution of instances may differ depending on their visiblity. One case where this arises is with incoherent instances

    module Main where

main :: IO ()
main = do
  let i :: Int
      i = 42
  putStrLn (m1 i)
  putStrLn (m2 i)

class C1 a where
  m1 :: a -> String

instance {-# INCOHERENT #-} C1 a where
  m1 _ = "C1 incoherent"

instance C1 Int where
  m1 = show

class C2 a where
  m2 :: a -> String

instance {-# INCOHERENT #-} C2 a where
  m2 _ = "C2 incoherent"

$(return [])

instance C2 Int where
  m2 = show

    Here, C1 and C2 are the same classes with nearly identical instances. The only significant differences between C1 and C2, aside from the minor name change, is that all of C1’s instances are defined within the same declaration group, whereas the C2 Int instance is put in a separate declaration group from the incoherent C2 a instance. This has an impact on the runtime behavior of the main function

    $ runghc Main.hs
42
C2 incoherent

    Note that m1 i returns "42", but m2 i returns "C2 incoherent". When each of these expressions are typechecked, GHC must figure out which C1 Int and C2 Int instances to use:

    1. When resolving the C1 Int instance, GHC discovers two possible instances in the same declaration group: the incoherent C1 a instance and the non-incoherent C1 Int instance. According to the instance search rules described in Overlapping instances, because there is exactly one non-incoherent instance to pick, GHC will choose the C1 Int instance. As a result, m1 i will be equivalent to show i (i.e., "42").
    2. When resolving the C2 Int instance, GHC only discovers one instance in the same declaration group: the incoherent C2 a instance. Note that GHC does not see the C2 Int instance, as that is in a later declaration group that is made separate by the intervening declaration splice. As a result, GHC will choose the C2 a instance, making m2 i equivalent to "C2 incoherent".

  • Expression quotations accept most Haskell language constructs. However, there are some GHC-specific extensions which expression quotations currently do not support, including

(Compared to the original paper, there are many differences of detail. The syntax for a declaration splice uses “$” not “splice”. The type of the enclosed expression must be Quote m => m [Dec], not [Q Dec]. Typed expression splices and quotations are supported.)

-fenable-th-splice-warnings

Template Haskell splices won’t be checked for warnings, because the code causing the warning might originate from a third-party library and possibly was not written by the user. If you want to have warnings for splices anyway, pass -fenable-th-splice-warnings.

6.13.2. Using Template Haskell

  • The data types and monadic constructor functions for Template Haskell are in the library Language.Haskell.TH.Syntax.

  • You can only run a function at compile time if it is imported from another module. That is, you can’t define a function in a module, and call it from within a splice in the same module. (It would make sense to do so, but it’s hard to implement.)

  • You can only run a function at compile time if it is imported from another module that is not part of a mutually-recursive group of modules that includes the module currently being compiled. Furthermore, all of the modules of the mutually-recursive group must be reachable by non-SOURCE imports from the module where the splice is to be run.

    For example, when compiling module A, you can only run Template Haskell functions imported from B if B does not import A (directly or indirectly). The reason should be clear: to run B we must compile and run A, but we are currently type-checking A.

  • If you are building GHC from source, you need at least a stage-2 bootstrap compiler to run Template Haskell splices and quasi-quotes. A stage-1 compiler will only accept regular quotes of Haskell. Reason: TH splices and quasi-quotes compile and run a program, and then looks at the result. So it’s important that the program it compiles produces results whose representations are identical to those of the compiler itself.

Template Haskell works in any mode (--make, --interactive, or file-at-a-time). There used to be a restriction to the former two, but that restriction has been lifted.

6.13.3. Viewing Template Haskell generated code

The flag -ddump-splices shows the expansion of all top-level declaration splices, both typed and untyped, as they happen. As with all dump flags, the default is for this output to be sent to stdout. For a non-trivial program, you may be interested in combining this with the -ddump-to-file flag (see Dumping out compiler intermediate structures. For each file using Template Haskell, this will show the output in a .dump-splices file.

The flag -dth-dec-file dumps the expansions of all top-level TH declaration splices, both typed and untyped, in the file M.th.hs for each module M being compiled. Note that other types of splices (expressions, types, and patterns) are not shown. Application developers can check this into their repository so that they can grep for identifiers that were defined in Template Haskell. This is similar to using -ddump-to-file with -ddump-splices but it always generates a file instead of being coupled to -ddump-to-file. The format is also different: it does not show code from the original file, instead it only shows generated code and has a comment for the splice location of the original file.

Below is a sample output of -ddump-splices

TH_pragma.hs:(6,4)-(8,26): Splicing declarations
  [d| foo :: Int -> Int
      foo x = x + 1 |]
======>
  foo :: Int -> Int
  foo x = (x + 1)

Below is the output of the same sample using -dth-dec-file

-- TH_pragma.hs:(6,4)-(8,26): Splicing declarations
foo :: Int -> Int
foo x = (x + 1)

6.13.4. A Template Haskell Worked Example

To help you get over the confidence barrier, try out this skeletal worked example. First cut and paste the two modules below into Main.hs and Printf.hs:

{- Main.hs -}
module Main where

-- Import our template "pr"
import Printf ( pr )

-- The splice operator $ takes the Haskell source code
-- generated at compile time by "pr" and splices it into
-- the argument of "putStrLn".
main = putStrLn ( $(pr "Hello") )


{- Printf.hs -}
module Printf where

-- Skeletal printf from the paper.
-- It needs to be in a separate module to the one where
-- you intend to use it.

-- Import some Template Haskell syntax
import Language.Haskell.TH

-- Describe a format string
data Format = D | S | L String

-- Parse a format string.  This is left largely to you
-- as we are here interested in building our first ever
-- Template Haskell program and not in building printf.
parse :: String -> [Format]
parse s   = [ L s ]

-- Generate Haskell source code from a parsed representation
-- of the format string.  This code will be spliced into
-- the module which calls "pr", at compile time.
gen :: Quote m => [Format] -> m Exp
gen [D]   = [| \n -> show n |]
gen [S]   = [| \s -> s |]
gen [L s] = stringE s

-- Here we generate the Haskell code for the splice
-- from an input format string.
pr :: Quote m => String -> m Exp
pr s = gen (parse s)

Now run the compiler,

$ ghc --make -XTemplateHaskell main.hs -o main

Run main and here is your output:

$ ./main
Hello

6.13.5. Template Haskell quotes and Rebindable Syntax

Rebindable syntax does not play well with untyped TH quotes: applying the rebindable syntax rules would go against the lax nature of untyped quotes that are accepted even in the presence of unbound identifiers (see %s18102). Applying the rebindable syntax rules to them would force the code that defines the said quotes to have all the necessary functions (e.g ifThenElse or fromInteger) in scope, instead of delaying the resolution of those symbols to the code that splices the quoted Haskell syntax, as is usually done with untyped TH. For this reason, even if a module has untyped TH quotes with RebindableSyntax enabled, GHC turns off rebindable syntax while processing the quotes. The code that splices the quotes is however free to turn on RebindableSyntax to have the usual rules applied to the resulting code.

Typed TH quotes on the other hand are perfectly compatible with the eager application of rebindable syntax rules, and GHC will therefore process any such quotes according to the rebindable syntax rules whenever the RebindableSyntax extension is turned on in the modules where such quotes appear.

6.13.6. Using Template Haskell with Profiling

Template Haskell relies on GHC’s built-in bytecode compiler and interpreter to run the splice expressions. The bytecode interpreter runs the compiled expression on top of the same runtime on which GHC itself is running; this means that the compiled code referred to by the interpreted expression must be compatible with this runtime, and in particular this means that object code that is compiled for profiling cannot be loaded and used by a splice expression, because profiled object code is only compatible with the profiling version of the runtime.

This causes difficulties if you have a multi-module program containing Template Haskell code and you need to compile it for profiling, because GHC cannot load the profiled object code and use it when executing the splices.

Fortunately GHC provides two workarounds.

The first option is to compile the program twice:

  1. Compile the program or library first the normal way, without -prof.

  2. Then compile it again with -prof, and additionally use -osuf p_o to name the object files differently (you can choose any suffix that isn’t the normal object suffix here). GHC will automatically load the object files built in the first step when executing splice expressions. If you omit the -osuf ⟨suffix⟩ flag when building with -prof and Template Haskell is used, GHC will emit an error message.

The second option is to add the flag -fexternal-interpreter (see Running the interpreter in a separate process), which runs the interpreter in a separate process, wherein it can load and run the profiled code directly. There’s no need to compile the code twice, just add -fexternal-interpreter and it should just work. (this option is experimental in GHC 8.0.x, but it may become the default in future releases).

6.13.7. Template Haskell Quasi-quotation

QuasiQuotes
Since:6.10.1

Enable Template Haskell Quasi-quotation syntax.

Quasi-quotation allows patterns and expressions to be written using programmer-defined concrete syntax; the motivation behind the extension and several examples are documented in “Why It’s Nice to be Quoted: Quasiquoting for Haskell” (Proc Haskell Workshop 2007). The example below shows how to write a quasiquoter for a simple expression language.

Here are the salient features

  • A quasi-quote has the form [quoter| string |].

    • The ⟨quoter⟩ must be the name of an imported quoter, either qualified or unqualified; it cannot be an arbitrary expression.
    • The ⟨quoter⟩ cannot be “e”, “t”, “d”, or “p”, since those overlap with Template Haskell quotations.
    • There must be no spaces in the token [quoter|.
    • The quoted ⟨string⟩ can be arbitrary, and may contain newlines.
    • The quoted ⟨string⟩ finishes at the first occurrence of the two-character sequence "|]". Absolutely no escaping is performed. If you want to embed that character sequence in the string, you must invent your own escape convention (such as, say, using the string "|~]" instead), and make your quoter function interpret "|~]" as "|]". One way to implement this is to compose your quoter with a pre-processing pass to perform your escape conversion. See the discussion in %s5348 for details.

  • A quasiquote may appear in place of

    • An expression
    • A pattern
    • A type
    • A top-level declaration

    (Only the first two are described in the paper.)

  • A quoter is a value of type Language.Haskell.TH.Quote.QuasiQuoter, which is defined thus:

    data QuasiQuoter = QuasiQuoter { quoteExp  :: String -> Q Exp,
                                 quotePat  :: String -> Q Pat,
                                 quoteType :: String -> Q Type,
                                 quoteDec  :: String -> Q [Dec] }

    That is, a quoter is a tuple of four parsers, one for each of the contexts in which a quasi-quote can occur.

  • A quasi-quote is expanded by applying the appropriate parser to the string enclosed by the Oxford brackets. The context of the quasi-quote (expression, pattern, type, declaration) determines which of the parsers is called.

  • Unlike normal declaration splices of the form $(...), declaration quasi-quotes do not cause a declaration group break. See Syntax for more information.

Warning

QuasiQuotes introduces an unfortunate ambiguity with list comprehension syntax. Consider the following,

let x = [v| v <- [0..10]]

Without QuasiQuotes this is parsed as a list comprehension. With QuasiQuotes this is parsed as a quasi-quote; however, this parse will fail due to the lack of a closing |]. See %s11679.

The example below shows quasi-quotation in action. The quoter expr is bound to a value of type QuasiQuoter defined in module Expr. The example makes use of an antiquoted variable n, indicated by the syntax 'int:n (this syntax for anti-quotation was defined by the parser’s author, not by GHC). This binds n to the integer value argument of the constructor IntExpr when pattern matching. Please see the referenced paper for further details regarding anti-quotation as well as the description of a technique that uses SYB to leverage a single parser of type String -> a to generate both an expression parser that returns a value of type Q Exp and a pattern parser that returns a value of type Q Pat.

Quasiquoters must obey the same stage restrictions as Template Haskell, e.g., in the example, expr cannot be defined in Main.hs where it is used, but must be imported.

{- ------------- file Main.hs --------------- -}
module Main where

import Expr

main :: IO ()
main = do { print $ eval [expr|1 + 2|]
          ; case IntExpr 1 of
              { [expr|'int:n|] -> print n
              ;  _              -> return ()
              }
          }


{- ------------- file Expr.hs --------------- -}
module Expr where

import qualified Language.Haskell.TH as TH
import Language.Haskell.TH.Quote

data Expr  =  IntExpr Integer
           |  AntiIntExpr String
           |  BinopExpr BinOp Expr Expr
           |  AntiExpr String
    deriving(Show, Typeable, Data)

data BinOp  =  AddOp
            |  SubOp
            |  MulOp
            |  DivOp
    deriving(Show, Typeable, Data)

eval :: Expr -> Integer
eval (IntExpr n)        = n
eval (BinopExpr op x y) = (opToFun op) (eval x) (eval y)
  where
    opToFun AddOp = (+)
    opToFun SubOp = (-)
    opToFun MulOp = (*)
    opToFun DivOp = div

expr = QuasiQuoter { quoteExp = parseExprExp, quotePat =  parseExprPat }

-- Parse an Expr, returning its representation as
-- either a Q Exp or a Q Pat. See the referenced paper
-- for how to use SYB to do this by writing a single
-- parser of type String -> Expr instead of two
-- separate parsers.

parseExprExp :: String -> Q Exp
parseExprExp ...

parseExprPat :: String -> Q Pat
parseExprPat ...

Now run the compiler:

$ ghc --make -XQuasiQuotes Main.hs -o main

Run “main” and here is your output:

$ ./main
3
1