On Feb 12, 2016, at 12:17 PM, Dominik Bollmann <dominikbollm...@gmail.com> wrote:
> > Hi, > > I'm currently trying to understand GHC's implementation of Template > Haskell and I've had the following two questions when reading upon > deSugar/DsMeta.hs and deSugar/dsExpr.hs (disclaimer: I'm a complete > newbie to GHC and do not know pretty much anything about its internals; > so please excuse silly questions). > > 1) As far as I understand DsMeta, its purpose is to desugar the contents > of TH quotation brackets, e.g. [| \x -> x |]. Given the HsExpr contents, > say `expr`, of a quotation bracket, it creates a CoreExpr > /representation/ of `expr` such that /evaluating/ this CoreExpr > representation yields a TH expression equivalent to `expr`. (This is > similar to how quotations are implemented in the original TH paper). This sounds like one indirection too many. The desugared quote will be a CoreExpr that, when run, will produce the TH AST that matches the quote. > > On the other hand, when instead writing the above TH quote, (i.e., [| \x > -> x |]), explicitly as > > newName "x" >>= (\x -> lamE (varP x) (varE x)) (*) > > there is no need to build a CoreExpr /representation/ because we already > got the TH value we're interested in; Hence, in this case DsExpr > desugars the above (*) directly to a CoreExpr that represents it. > > Is my understanding of the above correct? I tried to confirm it using > the following to splices: > > test1 :: Q Exp > test1 = [| \x -> x |] > > test2 :: Q Exp > test2 = do > x <- newName "x" > lamE [(varP x)] (varE x) > > However, dumping the desugared output of these splices using `-ddump-ds` > gives the exact same CoreExprS: > > test1 :: Q Exp > [LclIdX, Str=DmdType] > test1 = > Language.Haskell.TH.Syntax.bindQ > @ Name > @ Exp > (newName (GHC.CString.unpackCString# "x"#)) > (\ (x_a399 :: Name) -> > lamE > (GHC.Types.: @ PatQ (varP x_a399) (GHC.Types.[] @ PatQ)) > (varE x_a399)) > > test2 :: Q Exp > [LclIdX, Str=DmdType] > test2 = >>> = > @ Q > Language.Haskell.TH.Syntax.$fMonadQ > @ Name > @ Exp > (newName (GHC.CString.unpackCString# "x"#)) > (\ (x_a39a :: Name) -> > lamE > (GHC.Types.: @ PatQ (varP x_a39a) (GHC.Types.[] @ PatQ)) > (varE x_a39a)) > > Could anyone explain to me why both CoreExpr dumps are the same? > Shouldn't the first be a /representation/ that, only when run, yields > the second CoreExpr? I'm not sure I understand why you think they should be different. What GHC is doing here seems correct to me; I would expect both tests to yield the same result. Sorry I can't be more helpful here! Richard _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
