PUBLIC
Thanks, this is useful and it is starting to convince myself that there *is* a
documentation bug here.
It seems the big thing I was missing was the existence of `hsOverLitName`.
That's what returns `Data.String.fromString` for overloaded string literals.
But then, does that mean that the `ol_witness` field of the `OverLit` is not
used by the renamer at all? This contradicts the Note, or maybe I am reading it
too wishfully - maybe the intention of `ol_witness` is to be for "entertainment
purposes only", i.e. something provided by the parser for third-party tools
but not consumed by the renamer. Or maybe `ol_witness` is only to be used in
getting information *from* the renamer to the typechecker (note that the code
you pasted below doesn't use the input's `ol_witness` field for anything at
all), in which case instead of just fixing the docs, we should move
`ol_witness` from `HsOverLit` to `XOverLit GhcRn` and `XOverLit GhcTc`. I'm
happy to prepare a patch for this (code + Note) if you agree this is the
correct reading of the current code.
Thanks,
Gergo
From: Simon Peyton Jones <simo...@microsoft.com>
Sent: Monday, July 12, 2021 4:37 PM
To: Erdi, Gergo <gergo.e...@sc.com>
Cc: 'GHC' <ghc-devs@haskell.org>
Subject: [External] RE: Using overloaded syntax to avoid `base` dependency (RE:
Marking ParsedModule fragments as non-user-originating)
I don't really understand how my question fits into the 'bug report' bucket.
The quoted passage is not from the user manual, but rather, from a GHC Note
Only that GHC is doing something that you think is wrong - or at least not as
documented. If so, that's a bug. If not, the conversation is illuminating, and
more easily rediscovered later in the bug tracker.
I am not interested in end-to-end behaviour, but what actually happens GHC
phase by GHC phase. When is the reference to `fromString` introduced, when is
it resolved (by default to `Data.String.fromString`), does `RebindableSyntax`
allo me to replace not just `fromString`, but also `unpackCString#`?
I'm happy to help - but can I ask that when you think you understand, can you
submit a patch that clarifies the relevant Note(s), or adds one, so that the
Gergos of the future will find the answer laid out right where you tried to
find it?
In GHC.Rename.Pat
rnOverLit origLit
= do { opt_NumDecimals <- xoptM LangExt.NumDecimals
; let { lit@(OverLit {ol_val=val})
| opt_NumDecimals = origLit {ol_val = generalizeOverLitVal (ol_val
origLit)}
| otherwise = origLit
}
; let std_name = hsOverLitName val
; (from_thing_name, fvs1) <- lookupSyntaxName std_name
* hsOverLitName returns Data.String.fromString for string literals. That is
where fromString first appears.
* Then lookupSyntaxName just returns Data.String.fromString when
RebindableSyntax is off; or looks up "fromString" when RebindableSyntax is on.
When I say "Data.String.fromString" here, I mean the original name i.e. the
fromString defined in Data.String - not some possibly different entity that
happens to be in scope with the qualified name "Data.String.fromString".
Does that help?
From: Erdi, Gergo <gergo.e...@sc.com<mailto:gergo.e...@sc.com>>
Sent: 12 July 2021 09:21
To: Simon Peyton Jones <simo...@microsoft.com<mailto:simo...@microsoft.com>>
Cc: 'GHC' <ghc-devs@haskell.org<mailto:ghc-devs@haskell.org>>
Subject: RE: Using overloaded syntax to avoid `base` dependency (RE: Marking
ParsedModule fragments as non-user-originating)
PUBLIC
I don't really understand how my question fits into the 'bug report' bucket.
The quoted passage is not from the user manual, but rather, from a GHC Note. My
reading of that note was that if I write a string literal in a Haskell program,
and compile it with OverloadedStrings, it would parse into `HsLit _ (HsString _
fs)` with `HsOverLit _ (OverLit _ (HsIsString _ fs) "Data.String.fromString"`,
and then the renamer and the type checker would work from that. If this
understanding were correct, then I could generate parsed (and not yet
renamed/typechecked) code that is, instead, `HsOverLit _ (OverLit _ (HsIsString
_ fs) "myStringLitUnpackerFunction"`, and there would be no `fromString`
dependency. Yet, that's not what seems to happen.
Can you (or anyone else) go into more detail about how rebindable syntax
resolution and OverloadedStrings interacts in this particular case? I am not
interested in end-to-end behaviour, but what actually happens GHC phase by GHC
phase. When is the reference to `fromString` introduced, when is it resolved
(by default to `Data.String.fromString`), does `RebindableSyntax` allo me to
replace not just `fromString`, but also `unpackCString#`?
From: Simon Peyton Jones <simo...@microsoft.com<mailto:simo...@microsoft.com>>
Sent: Monday, July 12, 2021 3:32 PM
To: Erdi, Gergo <gergo.e...@sc.com<mailto:gergo.e...@sc.com>>
Cc: 'GHC' <ghc-devs@haskell.org<mailto:ghc-devs@haskell.org>>
Subject: [External] RE: Using overloaded syntax to avoid `base` dependency (RE:
Marking ParsedModule fragments as non-user-originating)
Gergo,
If you think you have uncovered a bug, could you submit a bug report on the
issue tracker, with a way to reproduce it? It's a bit hard to decode exactly
what is happening from what you say.
The user manual documentation doesn't say this in so many words (that's a bug),
but with OverloadedStrings, the literal "foo" is replaced by
Data.String.fromString "foo"
Guessing a bit, that is probably why GHC complains that it can't load
Data.String.fromString.
If in addition you want to use your own fromString, not the built-in one, then
you need to add RebindableSyntax.
Simon
From: Erdi, Gergo <gergo.e...@sc.com<mailto:gergo.e...@sc.com>>
Sent: 12 July 2021 08:13
To: Simon Peyton Jones <simo...@microsoft.com<mailto:simo...@microsoft.com>>
Cc: 'GHC' <ghc-devs@haskell.org<mailto:ghc-devs@haskell.org>>
Subject: Using overloaded syntax to avoid `base` dependency (RE: Marking
ParsedModule fragments as non-user-originating)
PUBLIC
OK so I tried out OverloadedStrings and it basically went as bad as I expected.
The documentation on `HsOverLit` is very promising: it points to the Note
[Overloaded literal witnesses], which states:
Note [Overloaded literal witnesses]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
*Before* type checking, the HsExpr in an HsOverLit is the
name of the coercion function, 'fromInteger' or 'fromRational'.
So that sounds great, right? It sounds like just before renaming, I should be
able to replace `HsLit _ (HsString _ fs)` with `HsOverLit _ (OverLit _
(HsIsString _ fs) unpack` with my own `unpack` function coming from my own
package, and everything would work out. Unfortunately, this is not what
happens: if I try getting this through the renamer, I get this error:
Failed to load interface for 'Data.String'
no unit id matching 'base' was found
Can't find interface-file declaration for variable fromString
Probable cause: bug in .hi-boot file, or inconsistent .hi file
Use -ddump-if-trace to get an idea of which file caused the error
So even though I am specifying my own coercion function, it is still looking
for `Data.String.fromString` which is not going to work, since I don't have
`base`. So either I am misunderstanding that Note, or it is simply out of date,
but in either case, this isn't going to be a viable route to going base-less.
Gergo
From: Erdi, Gergo
Sent: Tuesday, July 6, 2021 5:39 PM
To: Simon Peyton Jones <simo...@microsoft.com<mailto:simo...@microsoft.com>>
Cc: GHC <ghc-devs@haskell.org<mailto:ghc-devs@haskell.org>>
Subject: RE: Marking ParsedModule fragments as non-user-originating
PUBLIC
Thanks Simon!
Of course, you're right, it's the renamer, not the typechecker - I didn't
really check, just saw that "it happens during `typecheckModule`.
I'll look at the rebindable syntax stuff in detail, but at least for
OverloadedStrings, I already know that the problem will be that ultimately they
do go through the `String` type from `base`, and I need to use GHC baselessly.
This is a problem for two reasons:
* I can't implement `IsString` for `MyString`, because `IsString` is in
`base`
* Even if I made my own fake `base` with a fake `IsString` class, there is
nothing to put in the codomain of `fromString`: I *only* have `MyString`, not
`String`. And renaming `MyString to `String` in my fake `base` is not going to
cut it, since `String` is wired into GHC to be a type synonym for `[Char]`
(which `MyString` is not).
I foresee similar problems for OverloadedLists :/
Thanks,
Gergo
From: Simon Peyton Jones <simo...@microsoft.com<mailto:simo...@microsoft.com>>
Sent: Tuesday, July 6, 2021 5:08 PM
To: Erdi, Gergo <gergo.e...@sc.com<mailto:gergo.e...@sc.com>>
Cc: GHC <ghc-devs@haskell.org<mailto:ghc-devs@haskell.org>>
Subject: [External] RE: Marking ParsedModule fragments as non-user-originating
The typechecker now complains that the `ViewPatterns` language extension is not
turned on
I think it's the renamer:
rnPatAndThen mk p@(ViewPat _ expr pat)
= do { liftCps $ do { vp_flag <- xoptM LangExt.ViewPatterns
; checkErr vp_flag (badViewPat p) }
More generally, don't you just want OverloadedStrings or OverloadedLists?
You might want to read Note [Handling overloaded and rebindable constructs] in
GHC.Rename.Expr, and
Note [Rebindable syntax and HsExpansion] in GCH.Hs.Expr. These Notes describe
how GHC already does something similar to what you want. Maybe you can use
the same mechanism in your plugin.
Simon
From: ghc-devs
<ghc-devs-boun...@haskell.org<mailto:ghc-devs-boun...@haskell.org>> On Behalf
Of Erdi, Gergo via ghc-devs
Sent: 06 July 2021 09:08
To: ghc-devs@haskell.org<mailto:ghc-devs@haskell.org>
Subject: Marking ParsedModule fragments as non-user-originating
PUBLIC
Hi,
I'd like to hijack some syntax (like string literals or list patterns) for my
own use, and I thought a low-tech way of doing that is to transform the
ParsedModule before typechecking. For example, if I have a function `uncons ::
Array a -> Maybe (a, Array a)`, I can rewrite the pattern `[x1, x2, x3]` into
the view pattern `(uncons -> Just (x1, (uncons -> Just (x2, (uncons -> Just
(x3, (uncons -> Nothing)))))))` and let the normal GHC type checker take over
from here.
This is working for me so far, except for one problem: the typechecker now
complains that the `ViewPatterns` language extension is not turned on. I would
like to make the view patterns coming from my ParsedModule rewriter to be
exempt from this check (but of course still require the `ViewPatterns`
extension for user-originating code). Is there a way to do that? Or would I be
better off checking for user-originating view patterns myself before the
rewrite and then changing the `DynFlags` to always enable view patterns for
typechecking?
Thanks,
Gergo
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Where you have a Financial Markets relationship with Standard Chartered PLC,
Standard Chartered Bank and their subsidiaries (the "Group"), information on
the regulatory standards we adhere to and how it may affect you can be found in
our Regulatory Compliance Statement at https: //www.sc.com/rcs/ and Regulatory
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Insofar as this communication is not sent by the Global Research team and
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