Why doesn't ghci load this?
Hi, I've been making my way through the exercises in the book "Real World Haskell". Below is my solution to an exercise asking for an implementation of groupBy in term sof a foldr. I haven't tested it extensively but is seems to work. GHCI rejects it though, if I uncomment the type signature for the 'step' function. Could someone please help me to understand why that is? Thanks, Jeff myGroupBy :: (a -> a -> Bool) -> [a] -> [[a]] myGroupBy p xs = foldr step [[]] xs where -- step :: a -> [ [ a ] ] -> [ [ a ] ] step x acc@( ys : yss ) | null ys = [ x ] : [] | p x ( head ys ) = ( x : ys ) : yss | ( p x ( head ys ) ) == False = [ x ] : acc
On 2009 Mar 28, at 0:59, Jeff Lasslett wrote:
myGroupBy :: (a -> a -> Bool) -> [a] -> [[a]] myGroupBy p xs = foldr step [[]] xs where -- step :: a -> [ [ a ] ] -> [ [ a ] ] step x acc@( ys : yss ) | null ys = [ x ] : [] | p x ( head ys ) = ( x : ys ) : yss | ( p x ( head ys ) ) == False = [ x ] : acc
In Haskell98, the type "a" in step is not the same as the one in myGroupBy. If you use the ScopedTypeVariables extension (see the ghc manual; just adding the option won't work) you can write the type. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH
Brandon S. Allbery KF8NH
On 2009 Mar 28, at 0:59, Jeff Lasslett wrote:
myGroupBy :: (a -> a -> Bool) -> [a] -> [[a]] myGroupBy p xs = foldr step [[]] xs where -- step :: a -> [ [ a ] ] -> [ [ a ] ] step x acc@( ys : yss ) | null ys = [ x ] : []
I wonder whether that line does anything? foldr is defined like this: foldr step zero (x:xs) = step x (foldr step zero xs) So it seems to me that when the step function doesn't affect either the accumulator(=zero) or xs, then the step function won't have any effect on the result foldr produces. The return value from your 'null ys' guard does not affect xs, nor does it change the accumulator acc. So how does it affect the result of foldr?
participants (3)
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7stud -
Brandon S. Allbery KF8NH -
Jeff Lasslett