Negation

[S. Doaitse Swierstra]

One we start discussing syntax again it might be a good occasion to  
reformulate/make more precise a few points.

The following program is accepted by the Utrecht Haskell Compiler  
(here we took great effort to follow the report closely ;-} instead of  
spending our time on n+k patterns), but not by the GHC and Hugs.

module Main where

-- this is a (rather elaborate) definition of the number 1
one = let x=1 in x

-- this is a definition of the successor function using section notation
increment = ( one + )

-- but if we now unfold the definition of one we get a parser error in  
GHC
increment' = ( let x=1 in x  +  )

The GHC and Hugs parsers are trying so hard to adhere to the meta rule  
that bodies of let-expressions
extend as far as possible when needed in order to avoid ambiguity,  
that they even apply that rule when there is no ambiguity;
here we have  only a single possible parse, i.e. interpreting the  
offending expression as ((let x = 1 in ) +).

Yes, Haskell is both a difficult language to parse and to describe  
precisely.

Doaitse


On 8 feb 2010, at 17:18, Simon Peyton-Jones wrote:

> Folks
>
> Which of these definitions are correct Haskell?
>
> x1 = 4 + -5
> x2 = -4 + 5
> x3 = 4 - -5
> x4 = -4 - 5
> x5 = 4 * -5
> x6 = -4 * 5
>
> Ghc accepts x2, x4, x6 and rejects the others with a message like
> Foo.hs:4:7:
>   Precedence parsing error
>       cannot mix `+' [infixl 6] and prefix `-' [infixl 6] in the  
> same infix expression
>
> Hugs accepts them all.
>
> I believe that the language specifies that all should be rejected.  http://haskell.org/onlinereport/syntax-iso.html
>
>
> I think that Hugs is right here.  After all, there is no ambiguity  
> in any of these expressions.  And an application-domain user found  
> this behaviour very surprising.
>
> I'm inclined to start a Haskell Prime ticket to fix this language  
> definition bug.  But first, can anyone think of a reason *not* to  
> allow all the above?
>
> Simon
>
>
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