From f3399c446c7507d46d6cc550aa2fe7027dbc1b5b Mon Sep 17 00:00:00 2001 From: "simonpj@microsoft.com" Date: Fri, 18 Jan 2008 14:55:03 +0000 Subject: Add quasi-quotation, courtesy of Geoffrey Mainland This patch adds quasi-quotation, as described in "Nice to be Quoted: Quasiquoting for Haskell" (Geoffrey Mainland, Haskell Workshop 2007) Implemented by Geoffrey and polished by Simon. Overview ~~~~~~~~ The syntax for quasiquotation is very similar to the existing Template haskell syntax: [$q| stuff |] where 'q' is the "quoter". This syntax differs from the paper, by using a '$' rather than ':', to avoid clashing with parallel array comprehensions. The "quoter" is a value of type Language.Haskell.TH.Quote.QuasiQuoter, which contains two functions for quoting expressions and patterns, respectively. quote = Language.Haskell.TH.Quote.QuasiQuoter quoteExp quotePat quoteExp :: String -> Language.Haskell.TH.ExpQ quotePat :: String -> Language.Haskell.TH.PatQ TEXT is passed unmodified to the quoter. The context of the quasiquotation statement determines which of the two quoters is called: if the quasiquotation occurs in an expression context, quoteExp is called, and if it occurs in a pattern context, quotePat is called. The result of running the quoter on its arguments is spliced into the program using Template Haskell's existing mechanisms for splicing in code. Note that although Template Haskell does not support pattern brackets, with this patch binding occurrences of variables in patterns are supported. Quoters must also obey the same stage restrictions as Template Haskell; in particular, in this example quote may not be defined in the module where it is used as a quasiquoter, but must be imported from another module. Points to notice ~~~~~~~~~~~~~~~~ * The whole thing is enabled with the flag -XQuasiQuotes * There is an accompanying patch to the template-haskell library. This involves one interface change: currentModule :: Q String is replaced by location :: Q Loc where Loc is a data type defined in TH.Syntax thus: data Loc = Loc { loc_filename :: String , loc_package :: String , loc_module :: String , loc_start :: CharPos , loc_end :: CharPos } type CharPos = (Int, Int) -- Line and character position So you get a lot more info from 'location' than from 'currentModule'. The location you get is the location of the splice. This works in Template Haskell too of course, and lets a TH program generate much better error messages. * There's also a new module in the template-haskell package called Language.Haskell.TH.Quote, which contains support code for the quasi-quoting feature. * Quasi-quote splices are run *in the renamer* because they can build *patterns* and hence the renamer needs to see the output of running the splice. This involved a bit of rejigging in the renamer, especially concerning the reporting of duplicate or shadowed names. (In fact I found and removed a few calls to checkDupNames in RnSource that are redundant, becuase top-level duplicate decls are handled in RnNames.) --- docs/users_guide/flags.xml | 6 ++ docs/users_guide/glasgow_exts.xml | 138 ++++++++++++++++++++++++++++++++++++++ 2 files changed, 144 insertions(+) (limited to 'docs') diff --git a/docs/users_guide/flags.xml b/docs/users_guide/flags.xml index 344230262d..166ff113d0 100644 --- a/docs/users_guide/flags.xml +++ b/docs/users_guide/flags.xml @@ -742,6 +742,12 @@ dynamic + + + Enable quasiquotation. + dynamic + + Enable bang patterns. diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml index 9b4dc588d3..f22e6c9edf 100644 --- a/docs/users_guide/glasgow_exts.xml +++ b/docs/users_guide/glasgow_exts.xml @@ -290,6 +290,17 @@ documentation describes all the libraries that come with GHC. + + + + Enables quasiquotation (see ). + + Syntax stolen: + [:varid|. + + + @@ -4984,6 +4995,15 @@ Wiki page. the quotation has type Q Typ. + + A quasi-quotation can appear in either a pattern context or an + expression 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 . + + A name can be quoted with either one or two prefix single quotes: @@ -5158,6 +5178,124 @@ The basic idea is to compile the program twice: + Template Haskell Quasi-quotation +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. + + +In the example, the quasiquoter expr is bound to a value of +type Language.Haskell.TH.Quote.QuasiQuoter which contains two +functions for quoting expressions and patterns, respectively. The first argument +to each quoter is the (arbitrary) string enclosed in the Oxford brackets. The +context of the quasi-quotation statement determines which of the two parsers is +called: if the quasi-quotation occurs in an expression context, the expression +parser is called, and if it occurs in a pattern context, the pattern parser is +called. + + +Note that in the example we make 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. + + +In general, a quasi-quote has the form +[$quoter| string |]. +The quoter must be the name of an imported quoter; it +cannot be an arbitrary expression. The quoted string +can be arbitrary, and may contain newlines. + + +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. + + + + +{- 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 () + } + } + + +{- Expr.hs -} +module Expr where + +import qualified Language.Haskell.TH as TH +import Language.Haskell.TH.Quasi + +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 parseExprExp 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 + + + + -- cgit v1.2.1