Haskell Language Template Haskell & QuasiQuotes Syntax of Template Haskell and Quasiquotes
Example
Template Haskell is enabled by the -XTemplateHaskell GHC extension. This extension enables all the syntactic features further detailed in this section. The full details on Template Haskell are given by the user guide.
Splices
-
A splice is a new syntactic entity enabled by Template Haskell, written as
$(...), where(...)is some expression. -
There must not be a space between
$and the first character of the expression; and Template Haskell overrides the parsing of the$operator - e.g.f$gis normally parsed as($) f gwhereas with Template Haskell enabled, it is parsed as a splice. -
When a splice appears at the top level, the
$may be omitted. In this case, the spliced expression is the entire line. -
A splice represents code which is run at compile time to produce a Haskell AST, and that AST is compiled as Haskell code and inserted into the program
-
Splices can appear in place of: expressions, patterns, types, and top-level declarations. The type of the spliced expression, in each case respectively, is
Q Exp,Q Pat,Q Type,Q [Decl]. Note that declaration splices may only appear at the top level, whereas the others may be inside other expressions, patterns, or types, respectively.
Expression quotations (note: not a QuasiQuotation)
-
An expression quotation is a new syntactic entity written as one of:
[e|..|]or[|..|]-..is an expression and the quotation has typeQ Exp;[p|..|]-..is a pattern and the quotation has typeQ Pat;[t|..|]-..is a type and the quotation has typeQ Type;[d|..|]-..is a list of declarations and the quotation has typeQ [Dec].
-
An expression quotation takes a compile time program and produces the AST represented by that program.
-
The use of a value in a quotation (e.g.
\x -> [| x |]) without a splice corresponds to syntactic sugar for\x -> [| $(lift x) |], wherelift :: Lift t => t -> Q Expcomes from the class
class Lift t where
lift :: t -> Q Exp
default lift :: Data t => t -> Q Exp
Typed splices and quotations
-
Typed splices are similair to previously mentioned (untyped) splices, and are written as
$$(..)where(..)is an expression. -
If
ehas typeQ (TExp a)then$$ehas typea. -
Typed quotations take the form
[||..||]where..is an expression of typea; the resulting quotation has typeQ (TExp a). -
Typed expression can be converted to untyped ones:
unType :: TExp a -> Exp.
QuasiQuotes
-
QuasiQuotes generalize expression quotations - previously, the parser used by the expression quotation is one of a fixed set (
e,p,t,d), but QuasiQuotes allow a custom parser to be defined and used to produce code at compile time. Quasi-quotations can appear in all the same contexts as regular quotations. -
A quasi-quotation is written as
[iden|...|], whereidenis an identifier of typeLanguage.Haskell.TH.Quote.QuasiQuoter. -
A
QuasiQuoteris simply composed of four parsers, one for each of the different contexts in which quotations can appear:
data QuasiQuoter = QuasiQuoter { quoteExp :: String -> Q Exp,
quotePat :: String -> Q Pat,
quoteType :: String -> Q Type,
quoteDec :: String -> Q [Dec] }
Names
-
Haskell identifiers are represented by the type
Language.Haskell.TH.Syntax.Name. Names form the leaves of abstract syntax trees representing Haskell programs in Template Haskell. -
An identifier which is currently in scope may be turned into a name with either:
'eor'T. In the first case,eis interpreted in the expression scope, while in the second caseTis in the type scope (recalling that types and value constructors may share the name without amiguity in Haskell).
