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I find author to be right in he's statement, but wrong in type signature that can't do anything other from what he described. Actually, your type signature is better (in C++) since it doesn't contain concrete types, which means you cannot use any functions with those types.

Here's what you can do in Haskell:

     boo :: (Eq b) => Map a b -> b -> a
This would describe that all your function could possibly do inside is just using eq operation with values of map items.

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