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<p class="MsoNormal"><span style="font-size:12.0pt">I also recall that Idris and Elm have some do-syntax like this:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="Code">do { x <- e1<o:p></o:p></p>
<p class="Code"> ; Just y <- e2<o:p></o:p></p>
<p class="Code"> | Nothing -> exceptional-code<o:p></o:p></p>
<p class="Code"> ; etc<o:p></o:p></p>
<p class="Code"> ; etc }<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">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<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="Code">do { x <- e1<o:p></o:p></p>
<p class="Code"> ; mb_y <- e2<o:p></o:p></p>
<p class="Code"> ; case mb_y of<o:p></o:p></p>
<p class="Code"> Nothing -> exceptional-code<o:p></o:p></p>
<p class="Code"> Just y -> do { etc etc }<o:p></o:p></p>
<p class="Code"> }<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">I’d love this for Haskell, if someone felt like making a proposal. I do this kind of thing all the time!<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> ghc-devs [mailto:ghc-devs-bounces@haskell.org]
<b>On Behalf Of </b>Harendra Kumar<br>
<b>Sent:</b> 09 February 2018 02:43<br>
<b>To:</b> Brandon Allbery <allbery.b@gmail.com><br>
<b>Cc:</b> ghc-devs@haskell.org Devs <ghc-devs@haskell.org><br>
<b>Subject:</b> Re: DoAndIfThenElse<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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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.<o:p></o:p></p>
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-harendra<o:p></o:p></p>
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<o:p> </o:p></p>
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On 9 February 2018 at 08:08, Brandon Allbery <<a href="mailto:allbery.b@gmail.com" target="_blank">allbery.b@gmail.com</a>> wrote:<o:p></o:p></p>
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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)<o:p></o:p></p>
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<o:p> </o:p></p>
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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).<o:p></o:p></p>
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<o:p> </o:p></p>
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On Thu, Feb 8, 2018 at 9:24 PM, Harendra Kumar <<a href="mailto:harendra.kumar@gmail.com" target="_blank">harendra.kumar@gmail.com</a>> wrote:<o:p></o:p></p>
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Hi,<o:p></o:p></p>
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<o:p> </o:p></p>
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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?<o:p></o:p></p>
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<o:p> </o:p></p>
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thanks,<o:p></o:p></p>
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harendra<o:p></o:p></p>
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<o:p> </o:p></p>
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_______________________________________________<br>
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<span style="color:#888888"><br>
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<span class="hoenzb"><o:p></o:p></span></span></p>
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<o:p> </o:p></p>
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<span class="hoenzb"><span style="color:#888888">-- </span><o:p></o:p></span></p>
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<span style="color:#888888">brandon s allbery kf8nh sine nomine associates</span><o:p></o:p></p>
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<span style="color:#888888"><a href="mailto:allbery.b@gmail.com" target="_blank">allbery.b@gmail.com</a> <a href="mailto:ballbery@sinenomine.net" target="_blank">ballbery@sinenomine.net</a><o:p></o:p></span></p>
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<span style="color:#888888">unix, openafs, kerberos, infrastructure, xmonad <a href="https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fsinenomine.net&data=04%7C01%7Csimonpj%40microsoft.com%7Ceea21f8f64dd4051b39308d56f670635%7Cee3303d7fb734b0c8589bcd847f1c277%7C1%7C0%7C636537410654890985%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwifQ%3D%3D%7C-1&sdata=y0Yyny6wJfV47xbvLjyZFhyZoOWRBY4KQNJ3fwhaIZ8%3D&reserved=0" target="_blank">http://sinenomine.net</a><o:p></o:p></span></p>
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<o:p> </o:p></p>
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