[Haskell-cafe] Why doesn't TH expose its parser?

[Simon Peyton Jones]

| 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 at 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!
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