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 first example from that paper is set out below (A Template Haskell Worked Example) as a worked example to help get you started.
6.13.1. Syntax¶
- TemplateHaskell¶
- Implies:
- 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 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).
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.
A splice is written
$x
, wherex
is an arbitrary expression. There must be no space between the “$” and the expression.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]
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.This use of
$
overrides its meaning as an infix operator, just asM.x
overrides the meaning of.
as an infix operator. If you want the infix operator, put spaces around it.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 |]
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 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
NOTE: Currently typed splices may inhibit the unused identifier warning for identifiers in scope. See #16524.
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 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 literal7
. 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 functionf'7
is thus written' f'7
.''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.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
6.13.2. Levels and Stages¶
Template Haskell executes code at both compile time and runtime, which requires understanding two key concepts: levels and stages.
Levels are a concept the typechecker uses to ensure that code is well-staged - that is, the compiler can execute compile-time operations before runtime operations. Stages are the actual moments when code is compiled and executed. Levels are a semantic concept used by the typechecker, whilst stages are operational, a property of evaluation.
6.13.2.1. Understanding Levels¶
Every expression in a program exists at a specific integer level:
Level 0: Normal top-level declarations in a module
Level -1: Code inside a top-level splice (code that runs at compile time)
Level 1: Code inside a quotation (code that is quoted for runtime)
The level changes when entering quotes and splices:
Inside a quote
[| e |]
, the level increases by 1Inside a splice
$( e )
, the level decreases by 1
Thus, the level can be calculated as the number of surrounding quotes minus the number of surrounding splices. For example:
-- foo is at level 0
foo = $(let
-- bar is at level -1
bar = $(let
-- baz is at level -2
baz = [|
-- qux is at level -1
qux = [|
-- quux is at level 0
quux = [|
-- quuz is at level 1
quuz = 0
|]
|]
|]
in baz)
in bar)
Top-level splices (which define where compile-time evaluation happens) are characterized by having their body at a negative level.
Top-level declarations introduce variables at level 1.
Imports introduce variables at level 1.
Local variables are introduced at the level of their expression. For example, the
x
in [| let x = 0 in … |] is at level 2.
6.13.2.2. Cross-Stage Persistence¶
In normal Template Haskell, cross-stage persistence (CSP) allows identifiers to be used at levels different from where they were defined. There are two mechanisms for this:
Path-based persistence: This allows a global definition at one level to be used at a different level in two cases:
Any global identifier can be used at a later level (i.e. inside a quotation).
An imported identifier can be used at an earlier level (i.e. in a splice)
The
ImplicitStagePersistence
extension controls whether path-based persistence is enabled. It is enabled by default in all current language editions.Serialisation-based persistence: This allows locally-bound variables to be used at higher levels through the
Lift
typeclass:tardy x = [| x |] -- This is elaborated to [| $(lift x) |]
When the compiler sees a level error where a variable used one level higher than it is defined, it will automatically insert a
lift
to serialise the variable at the required level.This functionality is exposed to the user as the
Lift
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
Lift
is defined for most built-in types and can be derived using theDeriveLift
extension. See Deriving Lift instances for more information.
Path-based persistence explains why this code works:
module M where
suc :: Int -> Int
suc = (+1)
one :: Q Exp
one = [| \x -> suc x |] -- suc is used at level 1, defined at level 0
two = $(one) -- one is used at level -1, defined at level 0
With ExplicitLevelImports
and NoImplicitStagePersistence
,
path-based persistence is disabled, requiring explicit indication of which
identifiers can be used at which levels.
6.13.2.3. Stages and Compilation¶
While levels are a typechecker concept, stages refer to the actual moments when modules are compiled and executed:
Stage C (Compile time): Code that runs during compilation
Stage R (Runtime): Code that runs when the compiled program is executed
The compiler may need to compile code differently depending on the stage.
For example, if you are using -fno-code
, no code is needed for the R stage
but code generation will be needed for the C stage. If your compiler is dynamically
linked then the C stage code will need to be dynamically linked, but the R stage
may be statically linked.
The cross-stage persistence rules admitted by a language arise from assumptions
made about the stage structure. For GHC, with ImplicitStagePersistence
,
it must be assumed that a module will be available at all stages. This is a strong
requirement.
6.13.3. Declaration Groups¶
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…
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
.
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:
When resolving the
C1 Int
instance, GHC discovers two possible instances in the same declaration group: the incoherentC1 a
instance and the non-incoherentC1 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 theC1 Int
instance. As a result,m1 i
will be equivalent toshow i
(i.e.,"42"
).When resolving the
C2 Int
instance, GHC only discovers one instance in the same declaration group: the incoherentC2 a
instance. Note that GHC does not see theC2 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 theC2 a
instance, makingm2 i
equivalent to"C2 incoherent"
.
6.13.3.1. Miscellaneous other features¶
In this section the other features and issues of Template Haskell are discussed.
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 = [| \ $(mkPat) -> x : w |]
will fail with
z
being out of scope in the definition offoo
but it will not fail withw
being out of scope in the definition ofbar
. That will only happen whenbar
is spliced.A pattern quasiquoter may generate binders that scope over the right-hand side of a definition because these binders are in scope lexically. For example, given a quasiquoter
haskell
that parses Haskell, in the following code, they
in the right-hand side off
refers to they
bound by thehaskell
pattern quasiquoter, not the top-levely = 7
.y :: Int y = 7 f :: Int -> Int -> Int f n = \ [haskell|y|] -> y+n
The
TemplateHaskellQuotes
extension is considered safe under Safe Haskell whileTemplateHaskell
is not.Expression quotations accept most Haskell language constructs. However, there are some GHC-specific extensions which expression quotations currently do not support, including
(Compared to the original paper, there are many differences of detail.
The syntax for a declaration splice uses “$
” 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
.
6.13.4. Explicit Level Imports¶
The ExplicitLevelImports
extension, along with
ImplicitStagePersistence
, gives programmers fine-grained control
over which modules are needed at each stage of execution.
For a detailed description of the extension, see the paper Explicit Level Imports.
- ExplicitLevelImports¶
- Implies:
- Since:
9.14.1
Enable explicit level imports for Template Haskell, allowing programmers to specify which modules are needed at which level.
This introduces the
splice
andquote
import modifiers which allow a user to precisely express the level of identifiers introduced by an import.
- ImplicitStagePersistence¶
- Default:
on
- Since:
9.14.1
Allow identifiers to be used at different levels than where they’re defined, using path-based persistence.
6.13.4.1. Syntax and Usage¶
ExplicitLevelImports
adds two new import modifiers:
import splice M (...)
- imports identifiers at level -1 (for use in splices)import quote M (...)
- imports identifiers at level 1 (for use in quotations)import M (...)
- imports identifiers at level 0 (normal code)
The syntax supports both options for placement of the level keywords:
import splice M -- before the module name
import M splice -- after the module name
import splice qualified M as MB -- with qualified
import splice M qualified as MB -- with -XImportQualifiedPost
import M splice qualified as MB -- with -XImportQualifiedPost
6.13.4.2. Basic Examples¶
Explicit level imports allow you to be more precise about which modules are needed at which level.
{-# LANGUAGE TemplateHaskell #-}
module Main where
import Control.Lens.TH (makeLenses)
import OtherModule (someFunction)
data User = User { _name :: String, _age :: Int }
$(makeLenses ''User)
main = print (someFunction (User "John" 30))
In this version, both Control.Lens.TH
and OtherModule
are imported
normally. GHC must compile both modules before it can start type-checking Main,
because it can’t tell in advance which imports might be needed when evaluating
the makeLenses
splice. Even though only makeLenses
is actually used in
the splice, GHC must assume that any imported identifier might be needed.
If you use ExplicitLevelImports
, you can be more precise about which
modules are needed at which level. For example,
.. code-block:: haskell
{-# LANGUAGE TemplateHaskell, ExplicitLevelImports #-} module Main where
import splice Control.Lens.TH (makeLenses) import OtherModule (someFunction)
data User = User { _name :: String, _age :: Int }
$(makeLenses ‘’User)
main = print (someFunction (User “John” 30))
With explicit level imports, we’ve marked Control.Lens.TH
with the
splice
keyword, which tells GHC that this module is needed at compile-time
for evaluating splices. This provides GHC with crucial information:
Control.Lens.TH
must be compiled to object code before type-checkingMain
OtherModule
only needs to be type-checked beforeMain
, with code generation potentially happening in parallelControl.Lens.TH
won’t be needed at runtime (assuming there are no other references to it)
This distinction brings several benefits:
GHC doesn’t need to wait for
OtherModule
to be fully compiled before starting onMain
Control.Lens.TH
won’t be linked into the final executable since it’s only needed at compile-timeThe staging structure of the program is more explicit
Another example showing different import levels:
{-# LANGUAGE TemplateHaskell, ExplicitLevelImports #-}
module Advanced where
import splice A (makeFunction) -- Used in splices (level -1)
import B (normalFunction) -- Used in normal code (level 0)
import quote C (runtimeValue) -- Used in quotes (level 1)
-- This generates a function at compile time
$(makeFunction "generatedFunction")
-- This uses a normal function at runtime
result = normalFunction 42
-- This creates a quotation containing code that will use runtimeValue
quotation = [| runtimeValue * 2 |]
In this example, we’re explicitly marking each import with its intended level:
* A
provides code that runs at compile time (in splices)
* B
provides code that runs at normal runtime
* C
provides values that will be referenced in quoted code
6.13.4.3. Level Rules and Errors¶
With NoImplicitStagePersistence
:
Functions imported at level 0 can only be used at level 0
Functions imported with
splice
can only be used inside top-level splicesFunctions imported with
quote
can only be used inside quotes
Errors will occur if you use an identifier at the wrong level:
import splice A (foo) -- foo at level -1
import B (bar) -- bar at level 0
import quote C (baz) -- baz at level 1
x = $(foo 42) -- OK: foo used at level -1
y = $(bar 42) -- Error: bar imported at level 0 but used at level -1
z = [| baz 42 |] -- OK: baz used at level 1
w = [| bar 42 |] -- Error: bar imported at level 0 but used at level 1
6.13.4.4. Class Instances and Levels¶
Class instances are also subject to level checking. Instances must be available at the level where they’re used:
Instances from the current module are at level 0
Instances from normally imported modules are at level 0
Instances from splice-imported modules are at level -1
Instances from quote-imported modules are at level 1
Since classes are imported transitively, the typechecker ensures that there is a well-levelled path to access any instance. For example, if an instance is needed at level -1, then the instance must come from the transitive closure of splice imported modules.
6.13.4.5. Prelude Imports¶
The implicit Prelude
import only brings identifiers into scope at level 0.
If you need Prelude
functions in splices or quotes, you must explicitly
import them:
import splice Prelude (map, filter) -- Use these in splices
import quote Prelude (show, (+)) -- Use these in quotes
6.13.4.6. Notes and Limitations¶
Local definitions (those defined in the same module) are still subject to level rules - you can’t use a function in a splice if it’s defined in the same module
ExplicitLevelImports
works best when most Template Haskell usage is isolated to a few modulesDefining
Lift
instances requires special handling since the datatype must be available at both compile-time and runtime
6.13.5. 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 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.
6.13.6. 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)
6.13.7. 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
6.13.8. 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.
6.13.9. 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).
6.13.10. 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 level 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