Re: DoAndIfThenElse

[Christian Maeder]

S. Doaitse Swierstra schrieb:
[..]
 \begin{code}
 main = do
  if True then putStrLn 1;
  else putStrLn 2
 \end{code}

 This does also not work with hugs (; at the end)
 
 This does not work since now you have two ;'s; one because you wrote
 one and one because you did not indent the else. Allowing this
 additional ; was done to prevent confusion, and as you can see even more
 confusion pops up now;-{{ That is why I expressed my concerns about this
 grammar patch.
 
  Doaitse

Indeed, these semicolons are confusing. I think it should be possible to
allow then and else starting in the same column as if without
these ;s (in a do block).

Christian

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DoAndIfThenElse

[Christian Maeder]

Hi,

seeing Haskell 2010 and
http://hackage.haskell.org/trac/haskell-prime/wiki/DoAndIfThenElse

saying:
Compiler support ¶
GHC full (no flag)

I wonder why I still get a parse error (possibly incorrect
indentation) for:

\begin{code}
main = do
  if True then putStrLn 1
  else putStrLn 2
\end{code}

Can I try out this feature somehow?

Cheers Christian
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Re: DoAndIfThenElse

[David Virebayre]

On Thu, Nov 26, 2009 at 10:29 AM, Christian Maeder christian.mae...@dfki.de
 wrote:

I wonder why I still get a parse error (possibly incorrect
 indentation) for:

 \begin{code}
 main = do
  if True then putStrLn 1
  else putStrLn 2
 \end{code}


Isn't the proposal about :

\begin{code}
main = do
 if True then putStrLn 1
 ;else putStrLn 2
\end{code}

?

David.
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Re: DoAndIfThenElse

[Ian Lynagh]

Hi Christian,

On Thu, Nov 26, 2009 at 10:29:10AM +0100, Christian Maeder wrote:
 
 seeing Haskell 2010 and
 http://hackage.haskell.org/trac/haskell-prime/wiki/DoAndIfThenElse
 
 saying:
 Compiler support ¶
 GHC   full (no flag)
 
 I wonder why I still get a parse error (possibly incorrect
 indentation) for:
 
 \begin{code}
 main = do
   if True then putStrLn 1
   else putStrLn 2
 \end{code}
 
 Can I try out this feature somehow?

I can't see any support for it in GHC, even in the HEAD. The page claims
full (no flag) for every implementation, so I'd guess it's just a
boilerplate table that hasn't been properly filled in.


Thanks
Ian

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Re: DoAndIfThenElse

[Christian Maeder]

David Virebayre schrieb:
 Isn't the proposal about :
 
 \begin{code}
 main = do
  if True then putStrLn 1
  ;else putStrLn 2
 \end{code}

This should go through, too, but also does not for me according to
http://hackage.haskell.org/trac/haskell-prime/wiki/DoAndIfThenElse

cite
Change the syntax for conditionals to

exp - if exp1 [;] then exp2 [;] else exp3

i.e., add optional semicolons before then and else, making the above
example legal. This has been recently added to jhc, GHC and Hugs, and so
far it has not caused any problems.
/cite

But the main purpose of the proposal was to support the notation without
 ; and the indentation by at least one character.

I just do not see that it has been recently added to GHC
I've checked ghc-6.10.4 and 6.12.0.20091010

Cheers Christian

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