[Haskell-cafe] ghc-api Static Semantics?

Thomas Schilling nominolo at googlemail.com
Thu Jan 26 19:00:12 CET 2012

On 26 January 2012 16:33, JP Moresmau <jpmoresmau at gmail.com> wrote:

> Thomas, thank you for that explanation about the different type of
> identifiers in the different phases of analysis. I've never seen that
> information so clearly laid out before, can it be added to the wikis
> (in http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/API
> or http://www.haskell.org/haskellwiki/GHC/As_a_library maybe)? I think
> it would be helpful to all people that want to dive into the GHC API.

Will do.

> On a side note, I'm going to do something very similar in my
> BuildWrapper project (which is now the backend of the EclipseFP IDE
> plugins): instead of going back to the API every time the user
> requests to know the type of "something" in the AST, I'm thinking of
> sending the whole typed AST to the Java code. Maybe that's something
> Christopher could use. Both the BuildWrapper code and Thomas's scion
> code are available on GitHub, as they provide examples on how to use
> the GHC API.

I really don't think you want to do much work on the front-end as that will
just need to be duplicated for each front-end.  That was the whole point of
building Scion in the first place.  I understand, of course, that Scion is
not useful enough at this time.

Well, I currently don't have much time to work on Scion, but the plan is as

  - Scion becomes a multi-process architecture.  It has to be since it's
not safe to run multiple GHC sessions inside the same process.  Even if
that were possible, you wouldn't be able to, say, have a profiling compiler
and a release compiler in the same process due to how static flags work.
Separate processes have the additional advantage that you can kill them if
they use too much memory (e.g., because you can't unload loaded interfaces).

  - Scion will be based on Shake and GHC will mostly be used in one-shot
mode (i.e., not --make).  This makes it easier to handle preprocessed
files.  It also allows us to generate and update meta-information on
demand.  I.e., instead of parsing and typechecking a file and then caching
the result for the current file, Scion will simply generate meta
information whenever it (re-)compiles a source file and writes that meta
information to a file.  Querying or caching that meta information then is
completely orthogonal to generating it.  The most basic meta information
would be a type-annotated version of the compiled AST (possibly + warnings
and errors from the last time it was compiled).  Any other meta information
can then be generated from that.

 - The GHCi debugger probably needs to be treated specially.  There also
should be automatic detection of files that aren't supported by the
bytecode compiler (e.g., those using UnboxedTuples) and force compilation
to machine code for those.

 - The front-end protocol should be specified somewhere.  I'm thinking
about using protobuf specifications and then use ways to generate custom
formats from that (e.g., JSON, Lisp S-Expressions, XML?).  And if the
frontend supports protocol buffers, then it can use that and be fast.  That
also means that all serialisation code can be auto-generated.

I won't have time to work on this before the ICFP deadline (and only very
little afterwards), but Scion is not dead (just hibernating).

> JP
> On Thu, Jan 26, 2012 at 2:31 PM, Thomas Schilling
> <nominolo at googlemail.com> wrote:
> >
> >
> > On 26 January 2012 09:24, Christopher Brown <cmb21 at st-andrews.ac.uk>
> wrote:
> >> Hi Thomas,
> >>
> >> By static semantics I mean use and bind locations for every name in the
> >> AST.
> >
> > Right, that's what the renamer does in GHC.  The GHC AST is parameterised
> > over the type of identifiers used.  The three different identifier types
> > are:
> >
> > RdrName: is the name as it occurred in source code. This is the output of
> > the parser.
> > Name: is basically RdrName + unique ID, so you can distinguish two "x"s
> > bound at different locations (this is what you want). This is the output
> of
> > the renamer.
> > Id: is Name + Type information and consequently is the output of the type
> > checker.
> >
> > Diagram:
> >
> >    String  --parser-->  HsModule RdrName  --renamer-->  HsModule Name
> >  --type-checker-->  HsBinds Id
> >
> > Since you can't hook in-between renamer and type checker, it's perhaps
> more
> > accurately depicted as:
> >
> >    String  --parser-->  HsModule RdrName  --renamer+type-checker-->
> >  (HsModule Name,  HsBinds Id)
> >
> > The main reasons why it's tricky to use the GHC API are:
> >
> > You need to setup the environment of packages etc.  E.g., the renamer
> needs
> > to look up imported modules to correctly resolve imported names (or give
> a
> > error).
> > The second is that the current API is not designed for external use.  As
> I
> > mentioned, you cannot run renamer and typechecker independently, there
> are
> > dozens of invariants, there are environments being updated by the various
> > phases, etc.  For example, if you want to generate code it's probably
> best
> > to either generate HsModure RdrName or perhaps the Template Haskell API
> > (never tried that path).
> >
> >
> > / Thomas
> >
> > --
> > Push the envelope. Watch it bend.
> >
> --
> JP Moresmau
> http://jpmoresmau.blogspot.com/

Push the envelope. Watch it bend.
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