On Feb 12, 2016, at 12:17 PM, Dominik Bollmann
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