undefined type prob

James Grist writes: | I've got a prob with the Function toHeap. It return an | argument with an undefined type. Well, I think that's | got something to do with why I can`t get it to work. | OK, for it's input, it takes a list of some datatype, | so I try at the hugs console.... | | toHeap "hello" | | ERROR - Unresolved overloading | *** Type : Heap a => a Char | *** Expression : toHeap "hello" | | How do I get round this prob? Say which Heap instance you want. It won't automatically default to Tree, even though that's the only Heap instance in scope. toHeap "hello" :: Tree Char