Hey guys so here is my code on which I get the weird error of "Multiple Declarations of mirror". I have other functions before that but none of them are named mirror... Any ideas?
mirror :: BinTree a -> BinTree a
mirror = undefined
mirror (Node tL x tR) = Node x mirror tR mirror tL
Multiple definitions of a function must have the same number of arguments left of the equals sign. This is not required from a theory standpoint (note: one of the definitons could certainly be a lambda or return another function) but people seem to like it as such definitions typically indicate a bug.
Specifically, you have one definition with zero arguments:
mirror = undefined
And one definition with one argument:
mirror (Node tL x tR) = Node x mirror tR mirror tL
you probably want:
mirror _ = undefined
mirror (Node tL x tR) = Node x mirror tR mirror tL
undefined is often used as a placeholder as it allows type-checking of the rest of the code before putting in another case - in this case a case for some sort of Bin or Nil constructor.
This is not the issue with this specific example, but since this is the first result on Google for "multiple definitions Haskell", I figured I should contribute what the problem was for me:
If you are defining the function multiple times using pattern-matching with some of the arguments, all of the definitions must be consecutive. If there is other code in between them, they are considered separate definitions.
Example: the following is invalid because the definition of b divides the definitions of a:
frobnicate :: Bool -> String
frobnicate True = "foo"
b = "bar"
frobnicate False = b
Line 2 and line 3 have conflicting types: you've defined mirror to be the constant undefined, and then attempt to define it as a one-argument function. Removing line 2 should fix the problem; it's not clear why you wrote it in the first place.
undefined has type a, which matches any other type.
a -> b, i.e., one-argument functions.You have conflicting definitions for mirror. The first clause,
mirror = undefined
is a catch-all, so the definition is considered finished by the compiler. The next clause is then considered to start a new definition. You should remove the undefined line.
undefined is probably intended to signal "fill in" while making sure the program compiles. You'll have to remove it to get a working program.