Feed elements of a tuple to a function as arguments in Haskell?

Question

In my Haskell program, I want to use printf to format a list of tuples. I can map printf over a list to print out the values one at a time like this:

mapM_ (printf "Value: %d\n") [1,2,3,4]  Value: 1 Value: 2 Value: 3 Value: 4 

I want to be able to do something like this:

mapM_ (printf "Values: %d %d\n") [(1,100),(2,350),(3,600),(4,200)]  Values: 1 100 Values: 2 350 Values: 3 600 Values: 4 200 

But this passes a tuple to printf, not two separate values. How can I turn the tuple into two arguments for printf?

Answer 1

Function uncurry converts a two-argument (curried) function into a function on pairs. Here's its type signature:

uncurry :: (a -> b -> c) -> (a, b) -> c 

You need to use it on printf, like this:

mapM_ (uncurry $ printf "Values: %d %d\n") [(1,100),(2,350),(3,600),(4,200)] 

Another solution is to use pattern matching to deconstruct the tuple, like this:

mapM_ (\(a,b) -> printf "Values: %d %d\n" a b) [(1,100),(2,350),(3,600),(4,200)]