Re: DoAndIfThenElse

2018-02-22 Thread Niklas Hambüchen 
I've filed https://ghc.haskell.org/trac/ghc/ticket/14842 for it.

On 09/02/2018 10.24, Simon Peyton Jones via ghc-devs wrote:
> At very least the extension should be documented! Would you like to open
> a ticket for that?  And even offer a patch?
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RE: DoAndIfThenElse

2018-02-09 Thread Simon Peyton Jones via ghc-devs 
I also recall that Idris and Elm have some do-syntax like this:


do { x <- e1

 ; Just y <-  e2

  | Nothing -> exceptional-code

 ; etc

 ; etc }

That is, e2 :: blah -> IO (Maybe t),  we can pattern match on the expected Just 
case, but still provide code for the Nothing case.  That’s much better than


do { x <- e1

 ; mb_y <- e2

 ; case mb_y of

 Nothing -> exceptional-code

 Just y ->  do { etc etc }

}

I’d love this for Haskell, if someone felt like making a proposal. I do this 
kind of thing all the time!

Simon



From: ghc-devs [mailto:ghc-devs-boun...@haskell.org] On Behalf Of Harendra Kumar
Sent: 09 February 2018 02:43
To: Brandon Allbery <allber...@gmail.com>
Cc: ghc-devs@haskell.org Devs <ghc-devs@haskell.org>
Subject: Re: DoAndIfThenElse

Since I started programming in Haskell a few years ago I have been using 
if-then-else in that manner without indentation and I never knew about this 
extension. I thought this is how it works. It seems this is the default now. 
But, I remember encountering an error in an older compiler version once and 
then I figured the my style was accepted in newer compiler versions only.

-harendra

On 9 February 2018 at 08:08, Brandon Allbery 
<allber...@gmail.com<mailto:allber...@gmail.com>> wrote:
Huh. I wonder if a section went missing; seems like none of the extensions that 
alter or relax layout are documented currently. (AlternativeLayoutRule, 
AlternativeLayoutRuleTransitional, DoAndIfThenElse, NondecreasingIndentation, 
RelaxedLayout)

IIRC DoAndIfThenElse relaxes a condition implied by layout but that normally 
only matters in "do": that if you break it into multiple lines, the "then" and 
"else" must be indented farther than the "if" or layout will consider them 
distinct new expressions (and thereby syntax errors).

On Thu, Feb 8, 2018 at 9:24 PM, Harendra Kumar 
<harendra.ku...@gmail.com<mailto:harendra.ku...@gmail.com>> wrote:
Hi,

I recently found a mention of DoAndIfThenElse extension somewhere. I looked 
inside the ghc user guide and could not find any such extension. Then I looked 
in the ghc man page, no mention. I googled and found a very sparse references 
to it here and there. Then I tried using the extension with ghc and ghc seems 
to accept it. What's the story behind this, why is it not documented but 
accepted?

thanks,
harendra

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RE: DoAndIfThenElse

2018-02-09 Thread Simon Peyton Jones via ghc-devs 
At very least the extension should be documented! Would you like to open a 
ticket for that?  And even offer a patch?

Thanks for pointing this out.

Simon

From: ghc-devs [mailto:ghc-devs-boun...@haskell.org] On Behalf Of Harendra Kumar
Sent: 09 February 2018 02:43
To: Brandon Allbery <allber...@gmail.com>
Cc: ghc-devs@haskell.org Devs <ghc-devs@haskell.org>
Subject: Re: DoAndIfThenElse

Since I started programming in Haskell a few years ago I have been using 
if-then-else in that manner without indentation and I never knew about this 
extension. I thought this is how it works. It seems this is the default now. 
But, I remember encountering an error in an older compiler version once and 
then I figured the my style was accepted in newer compiler versions only.

-harendra

On 9 February 2018 at 08:08, Brandon Allbery 
<allber...@gmail.com<mailto:allber...@gmail.com>> wrote:
Huh. I wonder if a section went missing; seems like none of the extensions that 
alter or relax layout are documented currently. (AlternativeLayoutRule, 
AlternativeLayoutRuleTransitional, DoAndIfThenElse, NondecreasingIndentation, 
RelaxedLayout)

IIRC DoAndIfThenElse relaxes a condition implied by layout but that normally 
only matters in "do": that if you break it into multiple lines, the "then" and 
"else" must be indented farther than the "if" or layout will consider them 
distinct new expressions (and thereby syntax errors).

On Thu, Feb 8, 2018 at 9:24 PM, Harendra Kumar 
<harendra.ku...@gmail.com<mailto:harendra.ku...@gmail.com>> wrote:
Hi,

I recently found a mention of DoAndIfThenElse extension somewhere. I looked 
inside the ghc user guide and could not find any such extension. Then I looked 
in the ghc man page, no mention. I googled and found a very sparse references 
to it here and there. Then I tried using the extension with ghc and ghc seems 
to accept it. What's the story behind this, why is it not documented but 
accepted?

thanks,
harendra

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--
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 ballb...@sinenomine.net<mailto:ballb...@sinenomine.net>
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Re: DoAndIfThenElse

2018-02-08 Thread Harendra Kumar 
Since I started programming in Haskell a few years ago I have been using
if-then-else in that manner without indentation and I never knew about this
extension. I thought this is how it works. It seems this is the default
now. But, I remember encountering an error in an older compiler version
once and then I figured the my style was accepted in newer compiler
versions only.

-harendra

On 9 February 2018 at 08:08, Brandon Allbery  wrote:

> Huh. I wonder if a section went missing; seems like none of the extensions
> that alter or relax layout are documented currently.
> (AlternativeLayoutRule, AlternativeLayoutRuleTransitional,
> DoAndIfThenElse, NondecreasingIndentation, RelaxedLayout)
>
> IIRC DoAndIfThenElse relaxes a condition implied by layout but that
> normally only matters in "do": that if you break it into multiple lines,
> the "then" and "else" must be indented farther than the "if" or layout will
> consider them distinct new expressions (and thereby syntax errors).
>
> On Thu, Feb 8, 2018 at 9:24 PM, Harendra Kumar 
> wrote:
>
>> Hi,
>>
>> I recently found a mention of DoAndIfThenElse extension somewhere. I
>> looked inside the ghc user guide and could not find any such extension.
>> Then I looked in the ghc man page, no mention. I googled and found a very
>> sparse references to it here and there. Then I tried using the extension
>> with ghc and ghc seems to accept it. What's the story behind this, why is
>> it not documented but accepted?
>>
>> thanks,
>> harendra
>>
>> ___
>> ghc-devs mailing list
>> ghc-devs@haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>>
>>
>
>
> --
> brandon s allbery kf8nh   sine nomine
> associates
> allber...@gmail.com
> ballb...@sinenomine.net
> unix, openafs, kerberos, infrastructure, xmonad
> http://sinenomine.net
>
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Re: DoAndIfThenElse

2018-02-08 Thread Moritz Angermann 
Hi,

not sure if this helps.

testsuite/tests/parser/should_compile/DoAndIfThenElse.hs gives us
```
{-# LANGUAGE DoAndIfThenElse #-}

module DoAndIfThenElse where

foo :: IO ()
foo = do if True
 then return ()
 else return ()
```

and there is some other mention in 
libraries/bytestring/bench/wiki-haskell.html, which states:
```
Haskell 2010 adds the foreign function interface (FFI) to 
Haskell, allowing for bindings to other programming languages, fixes some syntax issues (changes in the formal grammar) and bans 
so-called "n-plus-k-patterns", that is, definitions of the form fact 
(n+1) = (n+1) * fact n are no longer allowed. It introduces the 
Language-Pragma-Syntax-Extension which allows for designating a Haskell source 
as Haskell 2010 or requiring certain extensions to the Haskell language. The 
names of the extensions introduced in Haskell 2010 are DoAndIfThenElse, 
HierarchicalModules, EmptyDataDeclarations, FixityResolution, 
ForeignFunctionInterface, LineCommentSyntax, PatternGuards, 
RelaxedDependencyAnalysis, LanguagePragma and NoNPlusKPatterns.[1]
```

in compiler/main/DynFlags.hs we find
```
languageExtensions (Just Haskell2010)
= [LangExt.ImplicitPrelude,
   LangExt.MonomorphismRestriction,
   LangExt.DatatypeContexts,
   LangExt.TraditionalRecordSyntax,
   LangExt.EmptyDataDecls,
   LangExt.ForeignFunctionInterface,
   LangExt.PatternGuards,
   LangExt.DoAndIfThenElse,
   LangExt.RelaxedPolyRec]
```

So, in Haskell2010, it's always on, and allows to write the above code. When we 
set
NoDoAndIfThenElse, we get
```
Unexpected semi-colons in conditional:
if True; then return (); else return ()
Perhaps you meant to use DoAndIfThenElse?
```

And then there's https://prime.haskell.org/wiki/DoAndIfThenElse.


Cheers,
 Moritz

> On Feb 9, 2018, at 10:24 AM, Harendra Kumar  wrote:
> 
> Hi,
> 
> I recently found a mention of DoAndIfThenElse extension somewhere. I looked 
> inside the ghc user guide and could not find any such extension. Then I 
> looked in the ghc man page, no mention. I googled and found a very sparse 
> references to it here and there. Then I tried using the extension with ghc 
> and ghc seems to accept it. What's the story behind this, why is it not 
> documented but accepted?
> 
> thanks,
> harendra
> ___
> ghc-devs mailing list
> ghc-devs@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs

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Re: DoAndIfThenElse

2018-02-08 Thread Brandon Allbery 
Huh. I wonder if a section went missing; seems like none of the extensions
that alter or relax layout are documented currently.
(AlternativeLayoutRule, AlternativeLayoutRuleTransitional, DoAndIfThenElse,
NondecreasingIndentation, RelaxedLayout)

IIRC DoAndIfThenElse relaxes a condition implied by layout but that
normally only matters in "do": that if you break it into multiple lines,
the "then" and "else" must be indented farther than the "if" or layout will
consider them distinct new expressions (and thereby syntax errors).

On Thu, Feb 8, 2018 at 9:24 PM, Harendra Kumar 
wrote:

> Hi,
>
> I recently found a mention of DoAndIfThenElse extension somewhere. I
> looked inside the ghc user guide and could not find any such extension.
> Then I looked in the ghc man page, no mention. I googled and found a very
> sparse references to it here and there. Then I tried using the extension
> with ghc and ghc seems to accept it. What's the story behind this, why is
> it not documented but accepted?
>
> thanks,
> harendra
>
> ___
> ghc-devs mailing list
> ghc-devs@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>
>


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