[Haskell-beginners] trouble using the foldl ....

[Sunil S Nandihalli]

I think I figured out..  I had to replace M.member with M.lookup..

Thanks,
Sunil.

On Thu, Aug 11, 2011 at 4:23 PM, Sunil S Nandihalli <
sunil.nandihalli at gmail.com> wrote:

> Hello everybody,
> When I compile a file containing ...
>
> plotAsString::(Integral a)=>[(a,a)]->a->String
> plotAsString locs ans = let xs = map fst locs
>                             ys = map snd locs
>                             locsmap = M.fromList (zip locs [0..])
>                             bufsize=10
>                             min' = ((\x -> x-bufsize) . minimum)
>                             max' = ((\x -> x+bufsize) . maximum)
>                             (bmin@(x0,y0),bmax@(x1,y1)) = ((min' xs,min'
> ys),(max' xs,max' ys))
>                             plotString = foldl (\curStr newRowId -> (foldl
> (\ccStr newColId -> (ccStr++(case (M.member (newRowId,newColId) locsmap) of
>
>                               (Just v) -> ("." ++ (show v) ++ ".")
>
>                               (Nothing) -> "   ")))
>
> curStr [y0..y1])) "" [x0..x1]
>                         in plotString
>
> I get the following error..
>
> main.hs:34:107:
>     Couldn't match expected type `Bool' with actual type `Maybe t0'
>     In the pattern: Just v
>     In a case alternative: (Just v) -> ("." ++ (show v) ++ ".")
>     In the second argument of `(++)', namely
>       `(case (M.member (newRowId, newColId) locsmap) of {
>           (Just v) -> ("." ++ (show v) ++ ".")
>           (Nothing) -> "   " })'
>
> can somebody help?
>
> Thanks,
> Sunil.
>
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