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
Sent: Monday, July 12, 2021 3:32 PM
To: Erdi, Gergo
Cc: 'GHC'
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 mailto:Gergo.Erdi@sc.com>
Sent: 12 July 2021 08:13
To: Simon Peyton Jones mailto:simonpj@microsoft.com>
Cc: 'GHC' 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 mailto:simonpj@microsoft.com>
Cc: GHC 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 mailto:simonpj@microsoft.com>
Sent: Tuesday, July 6, 2021 5:08 PM
To: Erdi, Gergo mailto:Gergo.Erdi@sc.com>
Cc: GHC 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 mailto:ghc-devs-bounces@haskell.org> On Behalf Of Erdi, Gergo via ghc-devs
Sent: 06 July 2021 09:08
To: ghc-devs@haskell.orgmailto: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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