7stud wrote:
Heinrich Apfelmus
writes: How about
import Data.Ord (comparing)
mySort = sortBy (comparing length)
I just finished chap. 3 of Real World Haskell, which doesn't even imports. It took me hours to figure out how to access sortBy. The book hasn't introduced "comparing", yet.
It's a handy little function for exactly this sort of thing. It's defined as comparing p x y = compare (p x) (p y) See also http://haskell.org/hoogle/?q=comparing
or at least
mySort = sortBy myCompare where myCompare x y = compare (length x) (length y)
Very nice. The book mentioned the compare function in chap. 2.
I have a question about that code: how come you don't have to specify a parameter for mySort, for example:
mySort xs = ...
And doesn't sortBy require two arguments?
sortBy :: (a -> a -> Ordering) -> [a] -> [a] (1) (2)
How come you can write it with only one argument?
You can supply arguments one at a time, this is called "currying". In other words, the expression sortBy myCompare is created by supplying one argument to sortBy and thus has one argument remaining. More specifically, it has the type sortBy myCompare :: [[a]] -> [[a]] Furthermore, we can simply set mySort = sortBy myCompare and the left hand side will be a function with one argument just like the right hand side is.
Finally, I'm wondering if anyone can explain why my let examples failed?
Your use of let in conjunction with where was not syntactically correct. While both are used for declaring new things, let is an expression while where is a declaration, these two don't mix. In particular, let must always have the form let x = ... y = ... ... in expr while where is used like this foo x y | ... = expr1 | ... = expr2 where a = ... b = ... ... Construction like let ... in where ... let ... in | ... do not exist, there is no expression after the in . I suggest consulting a syntax reference for Haskell or using where only for the time being. Regards, apfelmus -- http://apfelmus.nfshost.com