Re: GHC typecheck API

1 Dec 2017

      The report is timing how long a `:load` takes.

Sure, if/when I reproduce this on some open source module I'll make a
ticket with a test case!

On 1 December 2017 at 12:03, Simon Peyton Jones 
wrote:
...
There does seem to be a significant performance reduction with
-fdefer-type-errors:
https://github.com/commercialhaskell/intero/pull/
495#issuecomment-348474127
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That’s odd. I would expect zero effect if there aren’t any errors, and not
much even if ther are.  It’s not clear whether the report is to do with
compile time or runtime.
If you can boil down a test case, maybe a ticket would be good
*From:* Christopher Done [mailto:chrisdone@gmail.com]
*Sent:* 01 December 2017 12:01
*To:* Németh Boldizsár 
*Cc:* Simon Peyton Jones ; ghc-devs@haskell.org
*Subject:* Re: GHC typecheck API
We're currently experimenting with this for Intero. There does seem to be
a significant performance reduction with -fdefer-type-errors:
https://github.com/commercialhaskell/intero/pull/
495#issuecomment-348474127
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It's a classic cost benefit scenario: the benefits are much more type info
available, including go to definition and things like that being more
immediately up to date. The cost is having to wait more time.
I believe if there's a good compromise to this, it'll probably involve
sometimes enabling -fdefer-type-errors and most of the time turning it off
for immediate feedback.
On 1 December 2017 at 02:49, Németh Boldizsár  wrote:
Thank you for the suggestions!
Setting the -fdefer-type-errors flag is indeed a good way to do it (with
also adding Opt_DeferTypedHoles and Opt_DeferOutOfScopeVariables for other
kind of errors).
Boldizsár
2017.12.01. 4:12 keltezéssel, Christopher Done írta:
I suppose setting -fdefer-type-errors would also be handy in this scenario!
On Thu, 30 Nov 2017 at 15:37, Simon Peyton Jones via ghc-devs <
ghc-devs@haskell.org> wrote:
This sounds like a good project!
For the most part things look good:
* Most type checker errors arise from *type constraints*. The type
checkder tries to solve these, but returns an elaborated syntax tree (i.e.
typechecked, and annotated with types) even if constraint solving fails.
* Some renamer errors are like this, notably out-of-scope variables.
(They just show up as another constraint.)
However there is historical baggage.  Back in the beginning, most errors
were treated by throwing an exception in the typechecker monad; such
exceptions can be caught, so that we can get more than one error from the
file, but no syntax tree is returned.  Example
   let f = <expression> in <body>
If there was an error in <expression> we'd throw an exception, catch it at
the 'let', give 'f' the type
   f :: forall a. a
and continue to typecheck <body>.
The trouble with the exception stuff is that you don't get an elaborated
syntax tree.
So: I think you can get some of the way today, just by returning the tree
anyway even if there is an error to report.  But it'd take a bit more work
to make more and more errors into things that don't throw an exception.
(Look for failTc, failRn in thd code.)
I'm not very familiar with the GHC API for this part, but others will be.
I'm certain it can be improved, so rather than hacking around what is there
already, do propose and implement improvements.
Simon
| -----Original Message-----
| From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of Németh
| Boldizsár
| Sent: 30 November 2017 05:42
| To: ghc-devs@haskell.org
| Subject: GHC typecheck API
|
| Dear GHC developers,
|
| I'm developing a framework for development tools for Haskell. I use the
| GHC API to parse and typecheck the source files. I recently started to
| work on a quick-fix (automatic program correction) for Haskell source
| code (correcting parenthesis problems, like 'putStrLn "xxx" ++ show a'
| ==> 'putStrLn ("xxx" ++ show a)'). To do that I need to get the typed
| syntax tree even if the program contains type or rename errors. I could
| only do this by a nasty hack, adding a new TH module finalizer
| (tcg_th_modfinalizers) to extract the type checker's state before it
| fails. Is there a "correct" way to do this? I would not like this
| refactorings to be unusable if the GHC API changes.
|
| By the way, if you know of any similar attempt please let me know.
|
| Sincerely,
| Boldizsár Németh
| haskelltools.org
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