Treatment of unknown pragmas

Simon Peyton Jones simonpj at
Tue Oct 16 22:34:09 UTC 2018

I’m still not understanding what’s wrong with

{- HLINT blah blah -}

GHC will ignore it.  HLint can look at it.  Simple.

I must be missing something obvious.


From: ghc-devs <ghc-devs-bounces at> On Behalf Of Simon Marlow
Sent: 16 October 2018 21:44
To: Neil Mitchell <ndmitchell at>
Cc: ghc-devs <ghc-devs at>
Subject: Re: Treatment of unknown pragmas

I suggested to Neil that he add the {-# HLINT #-} pragma to GHC. It seemed like the least worst option taking into account the various issues that have already been described in this thread. I'm OK with adding HLINT; after all we already ignore OPTIONS_HADDOCK, OPTIONS_NHC98, a bunch of other OPTIONS, CFILES (a Hugs relic), and several more that GHC ignores.

We can either
(a) not protect people from mistyped pragmas, or
(b) protect people from mistyped pragma names, but then we have to bake in the set of known pragmas

We could choose to have a different convention for pragmas that GHC doesn't know about (as Ben suggests), but then of course we don't get any protection for mistyped pragma names when using that convention.


On Tue, 16 Oct 2018 at 21:12, Neil Mitchell <ndmitchell at<mailto:ndmitchell at>> wrote:
> A warning flag is an interesting way to deal with the issue. On the
> other hand, it's not great from an ergonomic perspective; afterall, this
> would mean that all users of HLint (and any other tool requiring special

Yep, this means every HLint user has to do an extra thing. I (the
HLint author) now have a whole pile of "how do I disable warnings in
Stack", and "what's the equivalent of this in Nix". Personally, it ups
the support level significantly that I wouldn't go this route.

I think it might be a useful feature in general, as new tools could
use the flag to prototype new types of warning, but I imagine once a
feature gets popular it becomes too much fuss.

> > I think it makes a lot of sense to have a standard way for third-parties
> > to attach string-y information to Haskell source constructs. While it's
> > not strictly speaking necessary to standardize the syntax, doing
> > so minimizes the chance that tools overlap and hopefully reduces
> > the language ecosystem learning curve.
> This sounds exactly like the existing ANN pragma, which is what I've wanted LiquidHaskell to move towards for a long time. What is wrong with using the ANN pragma?

Significant compilation performance penalty and extra recompilation.
ANN pragmas is what HLint currently uses.

>  I'm a bit skeptical of this idea. Afterall, adding cases to the
> lexer for every tool that wants a pragma seems quite unsustainable.

I don't find this argument that convincing. Given the list already
includes CATCH and DERIVE, the bar can't have been _that_ high to
entry. And yet, the list remains pretty short. My guess is the demand
is pretty low - we're just whitelisting a handful of additional words
that aren't misspellings.

Thanks, Neil
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