| What's the reason that there isn't a parseExp :: String -> Q Exp, | parseDecl, etc.? Good idea! I don't think it'd be too hard. The parser (in parser/Parser.y) already exposes parseExpression, parseDeclaration etc. But they generate HsSyn and we need TH syntax. We don’t currently have a way to do that. It's (B) in https://ghc.haskell.org/trac/ghc/wiki/TemplateHaskell/Conversions So there's some refactoring work to do here. Fiddly but not hard. Simon | -----Original Message----- | From: Haskell-Cafe [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | Christopher Done | Sent: 13 February 2015 21:32 | To: Haskell Cafe | Subject: [Haskell-cafe] Why doesn't TH expose its parser? | | What's the reason that there isn't a parseExp :: String -> Q Exp, | parseDecl, etc.? | | It's a pretty common use case to write some syntactic extension that | merely extends regular Haskell. We end up with the haskell-src-meta | package which doesn't support full GHC Haskell and doesn't know how to | parse infix applications. It doesn't give a strong story for | quasi-quotes to be useful for extensions to Haskell's syntax itself, | as opposed to just mini-DSLs. | | Here's a case in point: idiom brackets. You'd like to implement them like | this: | | [i|foo bar mu|] → foo <$> bar <*> mu | | You want it to be full Haskell inside, so foo could easilly be a case | or a let, etc., so a trivial way to do that would be to just parse the | string "foo bar mu" with parseExp and then do the above simple | transformation. Job done. | | In practice, we can't do this. Our best option is HSE, but it's not | ideal. Another way to do this is: | | $(idiom [|foo bar mu|]) | | And because [|…|] produces a Q Exp, you can just go ahead and do the | rewrite on the resulting Exp. Yay, we now have full GHC Haskell inside | the idiom bracket, but we've lost the syntactic convenience. Given | that [|…|] already does the String → Q Exp step, why not allow us to | re-use this functionality in quasi quotes? | | Ciao! | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe