[Haskell-cafe] The where-clause and guards

[Nicolas Wu]

Ugh, my formatting got eaten up by gmail. I just removed the where in
front of "m =", and aligned tat statment with your ballFold
definition. I would also align the first where statement with the case
bars of traceRay.

Nick

On Wed, Jul 21, 2010 at 8:53 AM, Nicolas Wu <nicolas.wu at gmail.com> wrote:
> As I understand the use of where clauses in [1], "A where clause is only allowed at the top level of a set of equations or case expression", would mean that the first "where" is scoping over the whole traceRay function: the "where m =" shouldn't be there, since there's already a where clause in scope.
>
> Try this:
>
> traceRay (x,y) r@((cx,cy,cz):n) (vx,vy,vz) iter
> | m > 100 = do
> color $ Color3 (sin vx) (cos vy) (cos vz)
> vertex $ Vertex2 x y
> | otherwise = do
> [boxx,boxy,boxz] <- boxFold [vx,vy,vz]
> (ballx,bally,ballz) <- ballFold (boxx,boxy,boxz)
> traceRay (x, y) r (2*ballx + cx, 2*bally + cy, 2*ballz + cz) (iter-1)
> where
> boxFold [] = return []
> boxFold (a:b)
> | a > 2 = do
> rem <- boxFold b
> return $ (2-a):rem
> | a < (-2) = do
> rem <- boxFold b
> return $ (-2-a):rem
> |otherwise = do
> rem <- boxFold b
> return $ (a):rem
> ballFold (x,y,z)
> | n < 0.5 = return (4*x, 4*y, 4*z)
> | n < 1 = return (x/(n*n), y/(n*n), z/(n*n))
> | otherwise = return (x, y, z)
> where n = sqrt $ x*x + y*y + z*z
> m = sqrt $ vx*vx + vy*vy + vz*vz
>
>
>
> [1] http://www.haskell.org/tutorial/patterns.html
>
> On Wed, Jul 21, 2010 at 8:38 AM, Eitan Goldshtrom <thesourceofx at gmail.com> wrote:
>> Well, perhaps you can help me figure out the problem with my exact program.
>> Just in case it matters, the program draws a Mandelbox via volumetric ray
>> casting. I can provide more information about the function, but I wouldn't
>> think it's necessary, since my problem is with parsing. The error I'm
>> getting is with the where-clause at the very bottom:
>>
>> traceRay (x,y) r@((cx,cy,cz):n) (vx,vy,vz) iter
>>    | m > 100        = do
>>        color $ Color3 (sin vx) (cos vy) (cos vz)
>>        vertex $ Vertex2 x y
>>    | otherwise      = do
>>        [boxx,boxy,boxz] <- boxFold [vx,vy,vz]
>>        (ballx,bally,ballz) <- ballFold (boxx,boxy,boxz)
>>        traceRay (x, y) r (2*ballx + cx, 2*bally + cy, 2*ballz + cz) (iter-1)
>>        where
>>            boxFold [] = return []
>>            boxFold (a:b)
>>                | a > 2        = do
>>                    rem <- boxFold b
>>                    return $ (2-a):rem
>>                | a < (-2)     = do
>>                    rem <- boxFold b
>>                    return $ (-2-a):rem
>>                |otherwise     = do
>>                    rem <- boxFold b
>>                    return $ (a):rem
>>            ballFold (x,y,z)
>>                | n < 0.5       = return (4*x, 4*y, 4*z)
>>                | n < 1         = return (x/(n*n), y/(n*n), z/(n*n))
>>                | otherwise     = return (x, y, z)
>>                where n = sqrt $ x*x + y*y + z*z
>>    where m = sqrt $ vx*vx + vy*vy + vz*vz
>>
>> On 7/21/2010 3:13 AM, Nicolas Wu wrote:
>>>
>>> There's nothing wrong with the use of your example, I'm guessing it's
>>> something in your ... that's leading to the parse error. This compiles
>>> just fine:
>>>
>>> f a b
>>>  | c>  1      = 1
>>>  | c<  1      = 2
>>>  | otherwise  = 3
>>>  where c = a+b
>>>
>>> Nick
>>>
>>
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>
>