[Haskell-beginners] data constructors

[Michael Mossey]

Daniel Fischer wrote:
> 
> import Data.Function (on)
> import Data.List
> 
> convert namedStaffs = map timeVertical verticals
>     where
>        nameTimedChords (name,tcs) = [(time,name,chord) | (time, chord) <- tcs]
>        timedNamedChords = sort . foldr merge [] . map nameTimedChords $ namedStaffs
>        fst3 (x,_,_) = x
>        verticals = groupBy ((==) `on` fst3) timedNamedChords
>        timeVertical v@((t,_,_):_) = (t,[(name,chord) | (_,name,chord) <- v])
> 
> ?

Hi Daniel,

Thanks, that is nice and I will learn a lot by studying it. However, a problem. It 
will be a big benefit to do this lazily because I only need to extract as many 
verticals as necessary to fit on one page of layout.

I realized, as you did, that it would simplify things to care only about 
NamedTimedChords:

type NamedTimedChord = (Name,Time,Chord)

Even though there is some redundancy to have every entry carry along its name and 
time, it is worth it for simplifying things. Here's what I have, to do this lazily 
and now caring only about NamedTimedChords:

toVerticals :: [[NamedTimedChord]] -> [[NamedTimedChord]]
toVerticals [] = []
toVerticals staves = firstVertical : toVerticals remainder
     where
       time3 (_,time,_) = time
       firstVertT = minimum $ map (time3 . head) staves
       usingStaves = [ s | s <- staves, time3 (head s) == firstVertT ]
       notUsingStaves = [ s | s <- staves, time3 (head s) /= firstVertT ]
       firstVertical = map head usingStaves
       remainder = leftOfUsing ++ notUsingStaves
       leftOfUsing = filter (not . null) (map tail usingStaves)