[Haskell-cafe] Re: Trouble with record syntax and classes

Thomas Nelson thn at cs.utexas.edu
Tue Feb 27 00:41:14 EST 2007

Thank you all for your advice so far.  I went back and tried to simplify 
my code, but I'm still stuck.  The basic idea I want is something like an 
arbitrary tree structure, where MetaSines are the branches containing 
Sines, and Sines are the leaves containing strings.  I want to recurse 
through the tree and pick up all the leaf strings, but only if that 
branch/leaf is "on" for that time.  Here's my second attempt at the code:

data ISine = Sine Integer Integer Integer String |
             MetaSine Integer Integer Integer [ISine]
letter (Sine _ _ _ l) = l
sub_sines (MetaSine _ _ _ xs) = xs
period p _ _ _ = p
offset _ o _ _ = o
threshold _ _ t _ = t

on :: Integer->ISine->Bool
on time (p o t x) = (mod (time-(offset p o t x)) (period p o t x)) < 
(threshold p o t x)

act time (MetaSine p o t s)
     |on time (p o t s)  = foldr (++) (map (act time) (sub_sines p o t s))
     |otherwise = []

act time (Sine p o t l)
     |on time p o t l = [letter p o t l]
     |otherwise = []

the on, period, offset, and threshold functions should work exactly the 
same for Sine and MetaSine, so I'd prefer to only write them once.  But 
the act function needs to differentiate: for a Sine, it just returns a 
singleton list if on is true, or an empty list otherwise; but for a 
Metasine, it needs to get the list of all the strings in Sine objects in 
it's sub_sines field.  It only does this if it's on. For example, let's 

       / |  \
      B  C    D
    / |     / | \
   E  F    G  H  I

If A, C, D, E, F and I are all on, only the strings of Sines C and I 
should be joined to make a to element list.  If everything is on, C, E, 
F, G, H, I will all be joined.

Anyway, the above code still doesn't work; there's a parse error for the 
definition of on.  I don't think my arguments are right somehow.  I 
appreciate your collective patience and expertise, and hope you can put me 
on the right track.


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