#1886: GHC API should preserve and provide access to comments
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Reporter: claus | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: GHC API | Version: 6.9
Severity: normal | Keywords:
Testcase: | Architecture: Unknown
Os: Unknown |
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one class of applications of the `GHC API` are program transformations
(refactoring, source to source optimisation, partial evaluation, ..) and
code layouters (pretty-print, 2html, syntax-colouring, ..). but, even
ignoring layout, parsing and pretty-printing with the `GHC API` does not
currently preserve the source (nor does it generate syntactically valid
code..).
consider this simple test: we want to parse a module, then pretty-print it
(we might want to adjust the layout, or switch between layout and explicit
braces). applying the attached code to itself gives this result:
{{{
$ /cygdrive/c/fptools/ghc/compiler/stage2/ghc-inplace -package ghc -e main
API_Layout.hs
module API where
import DynFlags
import GHC
import PprTyThing
import System.Process
import System.IO
import Outputable
import Data.Maybe
instance Num () where
[]
[]
{ fromInteger = undefined }
mode = CompManager
compileToCoreFlag = False
writer >| cmd = runInteractiveCommand cmd >>= \ (i, o, e, p) -> writer i
cmd |> reader = runInteractiveCommand cmd >>= \ (i, o, e, p) -> reader o
ghcDir = "c:/fptools/ghc/compiler/stage2/ghc-inplace --print-libdir"
|>
(fmap dropLineEnds . hGetContents)
where
dropLineEnds = filter (not . (`elem` "\r\n"))
main = defaultErrorHandler defaultDynFlags
$ do s <- newSession . Just =<< ghcDir
flags <- getSessionDynFlags s
(flags, _) <- parseDynamicFlags flags ["-package ghc"]
GHC.defaultCleanupHandler flags
$ do setSessionDynFlags s (flags {hscTarget = HscInterpreted})
addTarget s =<< guessTarget "API_Layout.hs" Nothing
load s LoadAllTargets
prelude <- findModule s (mkModuleName "Prelude") Nothing
usermod <- findModule s (mkModuleName "API") Nothing
setContext s [usermod] [prelude]
Just cm <- checkModule s (mkModuleName "API")
compileToCoreFlag
unqual <- getPrintUnqual s
printForUser stdout unqual $ ppr $ parsedSource cm
}}}
this has lost all comments, including pragmas, and is syntactically
invalid!
one suggestion, to avoid upsetting the rest of `ghc`, would be to preserve
the comments, with source locations, but to separate them from the main
abstract syntax tree. there would also need to be a way to link ast
fragments to comments, which might be slightly awkward. perhaps something
like:
{{{
-- was there a comment just preceeding the current AST fragment?
commentsBefore :: AST -> Maybe String
-- was there a comment immediately following the current AST fragment?
commentsAfter :: AST -> Maybe String
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1886>
GHC <http://www.haskell.org/ghc/>
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