Default module header `module Main where`

[Matthias Fischmann]

you're inside the module, so you have access to all top-level
bindings, not just to the export list.

the issue with the implicit `module Main(main) where` is that it
errors if main is not defined, even though that's a perfectly harmless
situation.

(i hope i got the question right?)


On Tue, May 16, 2017 at 05:53:17PM +0000, Adam Bergmark wrote:
> Date: Tue, 16 May 2017 17:53:17 +0000
> From: Adam Bergmark <adam at bergmark.nl>
> To: José Manuel Calderón Trilla <jmct at jmct.cc>, haskell-prime at haskell.org
> Subject: Re: Default module header `module Main where`
>
> GHCIs current behavior seems inconsistent with this, why is foo accessible
> and why don't I get an unused warning like i do if i have an explicit
> `module Main (main) where`?
>
> ```
> $ cat Main.hs
> main :: IO ()
> main = pure ()
>
> foo :: Int
> foo = 1
>
> $ ghci Main.hs -Wall
> GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
> Loaded GHCi configuration from /Users/adam.bergmark/.ghci
> [1 of 1] Compiling Main             ( Main.hs, interpreted )
> Ok, modules loaded: Main.
>
> λ foo
> 1
> ```
>
> On Tue, 16 May 2017 at 19:00 José Manuel Calderón Trilla <jmct at jmct.cc>
> wrote:
>
> > Agreed, this sounds sensible.
> >
> > Can anyone think of any unintended consequences?
> >
> > -Jose
> >
> > On Tue, May 16, 2017, at 09:50 AM, Iavor Diatchki wrote:
> >
> > That seems fairly reasonable to me.
> >
> > -Iavor
> >
> > On Tue, May 16, 2017 at 7:18 AM, Joachim Breitner <
> > mail at joachim-breitner.de> wrote:
> >
> > Hi,
> >
> > a very small proposal to be considered for Haskell':
> >
> > Currently, the report states
> >
> >     An abbreviated form of module, consisting only of the module body,
> >     is permitted. If this is used, the header is assumed to be ‘module
> >     Main(main) where’.
> >
> > I propose to change that to
> >
> >     An abbreviated form of module, consisting only of the module body,
> >     is permitted. If this is used, the header is assumed to be ‘module
> >     Main where’.
> >
> > The rationale is that a main-less main module is still useful, e.g.
> > when you are working a lot in GHCi, and offload a few extensions to a
> > separate file. Currently, tools like hdevtools will complain about a
> > missing main function when editing such a file.
> >
> > It would also work better with GHC’s -main-is flag, and avoid problems
> > like the one described in https://ghc.haskell.org/trac/ghc/ticket/13704
> >
> >
> > I don’t see any downsides. When compiling to a binary, implementations
> > are still able to detect that a Main module is not imported by any
> > other module and only the main function is used, and optimize as if
> > only main were exported.
> >
> > Greetings,
> > Joachim
> >
> >
> >
> > --
> > Joachim “nomeata” Breitner
> >   mail at joachim-breitner.de • https://www.joachim-breitner.de/
> >   XMPP: nomeata at joachim-breitner.de • OpenPGP-Key: 0xF0FBF51F
> >   Debian Developer: nomeata at debian.org
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> >
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> >

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