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!


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.


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, 

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:

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 


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brandon s allbery kf8nh                               sine nomine associates
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