I was thinking about the relationship between the wiki and the notes in
the GHC source.
Would it be possible to link directly to [compiler notes] in the GHC
source from the wiki, using hyperlinks? Right now, I'm seeing
references that look like: (See |Note [Constraint flavours]|.)
(I can see the motivation to include comments in the source, but I also
think that the wiki is more discoverable than the compiler source code.
So, in the interests of pursuing both approaches, it would be nice to be
able to link to notes FROM the wiki. I suppose one could include a
hyperlink to the file on github that contains the note...)
I'm not sure how much web infrastructure would be required to make
hyperlinks for notes...
-BenRI
On 11/8/21 5:35 AM, Simon Peyton Jones wrote:
Is there anywhere on the GHC wiki that explains how to interpret this
output, and says that the type and dictionary applications ARE there,
just not shown by '-ddump-tc'?
Perhaps it would be helpful to add some basic description of what
comes out of the typechecker to a page like this one? (below)
https://gitlab.haskell.org/ghc/ghc/-/wikis/commentary/compiler/hsc-main
<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Fghc%2Fghc%2F-%2Fwikis%2Fcommentary%2Fcompiler%2Fhsc-main&data=04%7C01%7Csimonpj%40microsoft.com%7Cab59b17d2f394945ad1e08d9a2b96c81%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637719740212483767%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=WZL1VADZPUlaOACd58K1XZO5MzPOKrfLFMSuBD%2FGW44%3D&reserved=0>
Yes it would! Would you care to start such a wiki page (a new one;
don’t just clutter up the one you point to)? You can write down
what you know. Don’t worry if you aren’t 100% sure – we can correct
it. And if you outright don’t know, leave a “What should I say here?”
note.
"This late desugaring is somewhat unusual. It is much more common to
desugar the program before typechecking, or renaming, because that
presents the renamer and typechecker with a much smaller language to
deal with. However, GHC's organisation means that
This note is now slightly out of date. We are now, very carefully,
doing some desugaring *before* typechecking. See
* Note [Handling overloaded and rebindable constructs] in
GHC.Rename.Expr
* Note [Rebindable syntax and HsExpansion] in GHC.Hs.Expr
You can and should point to these and similar Notes from the wiki page
you write. Indeed there may be some part of what you write that would
be better framed as Note in GHC’s source code.
Thanks!
Simon
PS: I am leaving Microsoft at the end of November 2021, at which point
simo...@microsoft.com <mailto:simo...@microsoft.com> will cease to
work. Use simon.peytonjo...@gmail.com
<mailto:simon.peytonjo...@gmail.com> instead. (For now, it just
forwards to simo...@microsoft.com.)
*From:*ghc-devs <ghc-devs-boun...@haskell.org> *On Behalf Of *Benjamin
Redelings
*Sent:* 08 November 2021 13:12
*To:* Richard Eisenberg <li...@richarde.dev>
*Cc:* ghc-devs@haskell.org
*Subject:* Re: Output language of typechecking pass?
Hi,
Questions:
1. It seems like this separation is actually necessary, in order to
apply generalization only to let arguments written by the programmer, and not
to let bindings introduced during desugaring. Is that right?
I don't think so. That is, if we did it all in one pass, I still think we
could get generalization right.
I guess I asked this question wrong. I mean to say, if we did the two
passes in the reverse order (desugaring first, followed by
typechecking), that would not work, right?
As the wiki says:
"This late desugaring is somewhat unusual. It is much more common to
desugar the program before typechecking, or renaming, because that
presents the renamer and typechecker with a much smaller language to
deal with. However, GHC's organisation means that
* error messages can display precisely the syntax that the user
wrote; and
* desugaring is not required to preserve type-inference properties.
"
2. Does the output of type checking contain type lambdas?
Yes. See below.
3. Does the type checking pass determine where to add dictionary
arguments?
Yes. See below.
4. Are there any other resources I should be looking at?
Yes. You want to enable -fprint-typechecker-elaboration (and possible
-fprint-explicit-coercions). With the former, you get to see all this stuff
you're looking for. It's normally suppressed so that the output resembles the
user's code.
I hope this helps!
Richard
Hmm... so, I think I see how this works now. I don't think
'-fprint-explicit-coercions' does anything here though.
$ ghc -ddump-tc Test2.hs -fprint-typechecker-elaboration
...
AbsBinds [a_a2hp] [$dNum_a2hB]
{Exports: [g <= g_a2hz
wrap: <>]
Exported types: g :: forall a. Num a => a -> a -> a
[LclId]
Binds: g x_aYk y_aYl = (y_aYl * x_aYk) + 1
Evidence: [EvBinds{[W] $dNum_a2hs = $dNum_a2hq
[W] $dNum_a2hw = $dNum_a2hq
[W] $dNum_a2hq = $dNum_a2hB}]}
...
The type and dictionary arguments are visible here (along with the
evidence bindings), but type and dictionary applications are only
visible if you use -ddump-tc-ast, which is a lot more verbose. (I
don't think there is another flag that shows these applications?)
Since I didn't initially know what "evidence" was, and there is
nothing to say that a_a2hp is a type lambda argument, this was pretty
opaque until I managed to read the tc-ast and the light went on.
I can see now that the type and dictionary arguments are added by
annotating the AST.
Is there anywhere on the GHC wiki that explains how to interpret this
output, and says that the type and dictionary applications ARE there,
just not shown by '-ddump-tc'?
Perhaps it would be helpful to add some basic description of what
comes out of the typechecker to a page like this one? (below)
https://gitlab.haskell.org/ghc/ghc/-/wikis/commentary/compiler/hsc-main
<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.haskell.org%2Fghc%2Fghc%2F-%2Fwikis%2Fcommentary%2Fcompiler%2Fhsc-main&data=04%7C01%7Csimonpj%40microsoft.com%7Cab59b17d2f394945ad1e08d9a2b96c81%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637719740212483767%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=WZL1VADZPUlaOACd58K1XZO5MzPOKrfLFMSuBD%2FGW44%3D&reserved=0>
-BenRI
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