On Wednesday 11 May 2011 17:39:39, michael rice wrote:
It's hard to improve on a 20 line Awk program for generating text but I thought it would be fun to investigate a Haskell solution. Why can't I cons an element onto an existing list? Michael Prelude Data.List Data.Map> insertWith (:) ("Moby", "Dick") "will" (fromList [(("Joe", "Blow"),["is"]), (("Moby", "Dick"),["may"])]) <interactive>:1:11: Occurs check: cannot construct the infinite type: a = [a] Expected type: a Inferred type: [a] In the first argument of `insertWith', namely `(:)' In the expression: insertWith (:) ("Moby", "Dick") "will" (fromList [(("Joe", "Blow"), ["is"]), (("Moby", "Dick"), ["may"])])Prelude Data.List Data.Map>
The type of insertWith is insertWith :: Ord k => (a -> a -> a) -> k -> a -> Data.Map.Map k a -> Data.Map.Map k a , so the combining function takes arguments of the same type, while (:) :: a -> [a] -> [a] Trying to use that for insertWith leads to the constraint a = [a], which gives rise to an infinite type error. (What would happen if the key is not yet in the Map, there's no way to pick a default value in general). Use `insertWith (++) key [value]' instead, or Data.Map.alter.