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
$x
, wherex
is an arbitrary expression. There must be no space between the “$” 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.- an expression; the spliced expression must have type
A expression quotation is written in Oxford brackets, thus:
[| ... |]
, or[e| ... |]
, where the ”...” is an expression; the quotation has typeQuote m => m Exp
.[d| ... |]
, where the ”...” is a list of top-level declarations; the quotation has typeQuote m => m [Dec]
.[t| ... |]
, where the ”...” is a type; the quotation has typeQuote m => m Type
.[p| ... |]
, where the ”...” is a pattern; the quotation has typeQuote m => m Pat
.
The
Quote
type class (Language.Haskell.TH.Syntax.Quote) is the minimal interface necessary to implement the desugaring of quotations. TheQ
monad is an instance ofQuote
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 ing
will causem
to be instantiated toQ
:h :: Int h = $(g) -- m ~ Q
A typed expression splice is written
$$x
, wherex
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
A typed expression quotation is written as
[|| ... ||]
, or[e|| ... ||]
, where the ”...” is an expression; if the ”...” expression has typea
, then the quotation has typeQuote m => Code m a
.It is possible to extract a value of type
m Exp
fromCode m a
using theunTypeCode :: 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:
[varid| ... |]
, where the ”...” is an arbitrary string; a full description of the quasi-quotation facility is given in Template Haskell Quasi-quotation.
A name can be quoted with either one or two prefix single quotes:
'f
has typeName
, and names the functionf
. Similarly'C
has typeName
and names the data constructorC
. In general'
⟨thing⟩ interprets ⟨thing⟩ in an expression context.A name whose second character is a single quote (sadly) cannot be quoted in 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 literal7
. There is no current escape mechanism in this (unusual) situation.''T
has typeName
, and names the type constructorT
. 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 thereify
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 promotex
into a value of typeQuote m => m Exp
. This functionality is exposed to the user as theLift
typeclass in theLanguage.Haskell.TH.Syntax
module. If a type has aLift
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., supposefoo
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., supposebar
is bound locally in the function definitionmkFoo bar = [| foo bar |]
), then GHC useslift
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 providesLift
instances for many common data types. Furthermore, it is possible to deriveLift
instances automatically by using theDeriveLift
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 writemodule 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 offoo
but it will not fail withw
being out of scope in the definition ofbar
. That will only happen whenbar
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, they
in the right-hand side off
refers to they
bound by thehaskell
pattern quasiquoter, not the top-levely = 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 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...- The splice
$(th1 ...)
would see the definition off
- 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). - The splice
$(blah1)
cannot refer to the functionw
-w
is part of a later declaration group, and thus invisible, similarly,$(blah1)
cannot see the definition ofh
(since it is part of the same declaration group as$(blah1)
. However, the splice$(blah1)
can see the definition off
(since it is in the immediately preceding declaration group). - The splice
$(th2 ...)
would see the definition off
, all the bindings created by$(th1 ...)
, the definition ofh
and all bindings created by[qq|blah|]
(they are all in previous declaration groups). - The body of
h
can refer to the functionk
appearing on the other side of the declaration quasiquoter, as quasiquoters do not cause a declaration group to be broken up. - The
qq
quasiquoter would be able to see the definition off
from the preceding declaration group, but not the definitions ofh
ork
, or any definitions from subsequent declaration groups. - 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
- The splice
$(th1 ...)
cannot refer toD
- it is in the same declaration group. - The declaration group containing
D
is terminated by the empty top-level declaration splice$(return [])
(recall,Q
is a Monad, so we may simplyreturn
the empty list of declarations). - Since the declaration group containing
D
is in the previous declaration group, the splice$(th2 ...)
can refer toD
.
- The splice
Expression quotations accept most Haskell language constructs. However, there are some GHC-specific extensions which expression quotations currently do not support, including
(Compared to the original paper, there are many differences of detail.
The syntax for a declaration splice uses “$
” 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 #18102). 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:
Compile the program or library first the normal way, without
-prof
.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 #5348 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
#11679.
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