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). 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? If anyone could help me understanding this, it'd be great! Thanks, Dominik.