RE: Understanding DsMeta module
| Thanks. That helps but I still don't understand why the calls are | delegated to template-haskell library. Couldn't all of this be done | locally? No. If you have f :: Int - Q Exp then f is a function that, when run, produces a data structure that is the syntax tree (in the data type of Language.Haskell.TH) of some expression. Later, in some other module entirely, you may call f, thus foo = $(f 5) Now GHC dynamically links the module that defines f, executes f's code, which returns a syntax tree (in the data type of Language.Haskell.TH). This is converted to HsSyn and replaces the $(f 5). So the code for f must be code that, when executed, produces a data structure. The business of DsMeta is to produce such code. If, once you grok this, you'd like to add a page to the GHC Commentary, to explain it, that would be great. It does take a while to get your head around. Simon | -Original Message- | From: ghc-devs [mailto:ghc-devs-boun...@haskell.org] On Behalf Of Jan | Stolarek | Sent: 17 December 2014 19:34 | To: Richard Eisenberg | Cc: ghc-devs@haskell.org | Subject: Re: Understanding DsMeta module | | Thanks. That helps but I still don't understand why the calls are | delegated to template-haskell library. Couldn't all of this be done | locally? | | Janek | | Dnia środa, 17 grudnia 2014, Richard Eisenberg napisał: | On Dec 17, 2014, at 12:29 PM, Jan Stolarek jan.stola...@p.lodz.pl | wrote: |Why not implement repPlainTV like this: ? | |repPlainTV :: Core TH.Name - DsM (Core TH.TyVarBndr) repPlainTV |(MkC nm) = return $ MkC (TH.PlainTV nm) | | In short, that's ill typed. We have | |newtype Core a = MkC CoreExpr | | The idea behind this type is that its (phantom) type parameter | tracks | the type of the expression stored within. Of course, the thing | within | is always just a core expression. TH.PlainTV takes a TH.Name and | produces a TH.TyVarBndr. But, nm is a CoreExpr and MkC is expecting | a | CoreExpr, so your suggestion wouldn't type check. | | The higher-level answer is that you're mixing levels. The goal in | DsMeta is | *not* to create the TH AST. It's to create *core expressions* that | create the TH AST. | | Does this help? | | Richard | | | ___ | ghc-devs mailing list | ghc-devs@haskell.org | http://www.haskell.org/mailman/listinfo/ghc-devs ___ ghc-devs mailing list ghc-devs@haskell.org http://www.haskell.org/mailman/listinfo/ghc-devs
Re: Understanding DsMeta module
On Dec 17, 2014, at 12:29 PM, Jan Stolarek jan.stola...@p.lodz.pl wrote: Why not implement repPlainTV like this: ? repPlainTV :: Core TH.Name - DsM (Core TH.TyVarBndr) repPlainTV (MkC nm) = return $ MkC (TH.PlainTV nm) In short, that's ill typed. We have newtype Core a = MkC CoreExpr The idea behind this type is that its (phantom) type parameter tracks the type of the expression stored within. Of course, the thing within is always just a core expression. TH.PlainTV takes a TH.Name and produces a TH.TyVarBndr. But, nm is a CoreExpr and MkC is expecting a CoreExpr, so your suggestion wouldn't type check. The higher-level answer is that you're mixing levels. The goal in DsMeta is *not* to create the TH AST. It's to create *core expressions* that create the TH AST. Does this help? Richard ___ ghc-devs mailing list ghc-devs@haskell.org http://www.haskell.org/mailman/listinfo/ghc-devs
Re: Understanding DsMeta module
Thanks. That helps but I still don't understand why the calls are delegated to template-haskell library. Couldn't all of this be done locally? Janek Dnia środa, 17 grudnia 2014, Richard Eisenberg napisał: On Dec 17, 2014, at 12:29 PM, Jan Stolarek jan.stola...@p.lodz.pl wrote: Why not implement repPlainTV like this: ? repPlainTV :: Core TH.Name - DsM (Core TH.TyVarBndr) repPlainTV (MkC nm) = return $ MkC (TH.PlainTV nm) In short, that's ill typed. We have newtype Core a = MkC CoreExpr The idea behind this type is that its (phantom) type parameter tracks the type of the expression stored within. Of course, the thing within is always just a core expression. TH.PlainTV takes a TH.Name and produces a TH.TyVarBndr. But, nm is a CoreExpr and MkC is expecting a CoreExpr, so your suggestion wouldn't type check. The higher-level answer is that you're mixing levels. The goal in DsMeta is *not* to create the TH AST. It's to create *core expressions* that create the TH AST. Does this help? Richard ___ ghc-devs mailing list ghc-devs@haskell.org http://www.haskell.org/mailman/listinfo/ghc-devs
Re: Understanding DsMeta module
But you need an expression that, say, produces a PlainTV. Where are you going to find an expression that does this without using the template-haskell library? On Dec 17, 2014, at 2:33 PM, Jan Stolarek jan.stola...@p.lodz.pl wrote: Thanks. That helps but I still don't understand why the calls are delegated to template-haskell library. Couldn't all of this be done locally? Janek Dnia środa, 17 grudnia 2014, Richard Eisenberg napisał: On Dec 17, 2014, at 12:29 PM, Jan Stolarek jan.stola...@p.lodz.pl wrote: Why not implement repPlainTV like this: ? repPlainTV :: Core TH.Name - DsM (Core TH.TyVarBndr) repPlainTV (MkC nm) = return $ MkC (TH.PlainTV nm) In short, that's ill typed. We have newtype Core a = MkC CoreExpr The idea behind this type is that its (phantom) type parameter tracks the type of the expression stored within. Of course, the thing within is always just a core expression. TH.PlainTV takes a TH.Name and produces a TH.TyVarBndr. But, nm is a CoreExpr and MkC is expecting a CoreExpr, so your suggestion wouldn't type check. The higher-level answer is that you're mixing levels. The goal in DsMeta is *not* to create the TH AST. It's to create *core expressions* that create the TH AST. Does this help? Richard ___ ghc-devs mailing list ghc-devs@haskell.org http://www.haskell.org/mailman/listinfo/ghc-devs
