Re: Unprincipled defaults

[David Feuer]

Whoa. I was far too confident that wouldn't work!

On Sep 2, 2016 12:39 PM, "Richard Eisenberg"  wrote:

> I feel like I must be missing something, but what's wrong with
>
> class Semigroup1 f where
>   op :: f a -> f a -> f a
>
>   default op :: Monoid (f a) => f a -> f a -> f a
>   op = (<>)
>
>
> ? Does that do what you like?
>
> Richard
>
>
> On Sep 1, 2016, at 11:15 PM, David Feuer  wrote:
>
> On occasion, it can be useful to have default definitions that don't
> typecheck even with DefaultSignatures. It would be nice to be able to use
> them anyway. For example, if we have
>
> class Semigroup1 f where
>   op :: f a -> f a -> f a
>
> then we could, hypothetically, give a default definition for (<>):
>
>   default (<>) :: Semigroup1 f => f a -> f a -> f a
>   (<>) = op
>
> But we can't give a default definition
>
>   op = (<>)
>
> because there's no way to write its signature. However, for any F with
>
> instance Semigroup (F a) where ...
>
> the definition op = (<>) is perfectly fine.
>
> Would it be possible to offer a completely wild defaulting mechanism
> allowing *any expression* as a default, and only checking its type and
> compiling it when it's actually used?
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>
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Re: Build GHC on Windows using Hadrian and Stack

[Edward Z. Yang]

Nice work!

Edward

Excerpts from Andrey Mokhov's message of 2016-09-02 02:45:58 +:
> Hi all,
> 
> Summary: Building GHC on Windows using Hadrian and Stack got even simpler - 
> see https://github.com/snowleopard/hadrian/blob/master/doc/windows.md.
> 
> 
> To build GHC on Windows you usually need to jump through a lot of hoops [1], 
> which may be confusing even for experienced GHC developers [2].
> 
> Hadrian to the rescue!
> 
> Hadrian, a new build system for GHC that I've been developing [3], is written 
> in Haskell and can therefore be built and run via Stack that can install 
> appropriate bootstrapping GHC and MSYS2 environment in an automated and 
> robust way. This was first pointed out by Neil Mitchell [4], and I've 
> recently simplified build instructions even further. Here are all the steps:
> 
> git clone --recursive git://git.haskell.org/ghc.git
> cd ghc
> git clone git://github.com/snowleopard/hadrian
> cd hadrian
> stack setup
> stack exec -- pacman -S autoconf automake-wrapper make patch tar --noconfirm
> stack build
> stack exec hadrian -- --directory ".." -j --flavour=quickest
> 
> See more details here: 
> https://github.com/snowleopard/hadrian/blob/master/doc/windows.md.
> 
> NB: Hadrian can build Stage2 GHC, but there are still many limitations [5]. 
> Help make it better!
> 
> [1] https://ghc.haskell.org/trac/ghc/wiki/Building/Preparation/Windows
> [2] https://mail.haskell.org/pipermail/ghc-devs/2016-June/012340.html
> [3] https://github.com/snowleopard/hadrian
> [4] http://neilmitchell.blogspot.co.uk/2016/03/compiling-ghc-on-windows.html
> [5] https://github.com/snowleopard/hadrian#current-limitations
> 
> Cheers,
> Andrey
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