mapValues is lazy

repl.scala

scala> def foo = { val x = Map[Any,Any](() -> ()).mapValues(_ => ???); () }
foo: Unit

scala> foo

scala> def foo = { val x = Map[Any,Any](() -> ()).map(_ => ???); () }
foo: Unit

scala> foo
scala.NotImplementedError: an implementation is missing
	at scala.Predef$.$qmark$qmark$qmark(Predef.scala:252)
	...

This example made me poke around Haskell's Data.Map:

ghci>import Data.Map as M
ghci> let g = M.map (+ 5) $ M.singleton (\_ -> error) 10
ghci> M.size g
1
ghci> let res = M.map (+ 5) g
ghci> :t res
res :: Num b => Map (t -> [Char] -> a) b