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

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?

Answers


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)]

mapM_ (\(x,y) -> printf "Value: %d %d\n" x y) [(1,100),(2,350),(3,600),(4,200)]

A type-safe alternative to Text.Printf is the formatting package. Text.Printf.printf does not ensure at compile-time that the number of formatting parameters aligns with the number of arguments and their types. Read Chris Done's article, What's wrong with printf? for examples.

An example usage:

{-# LANGUAGE OverloadedStrings #-}
import Formatting

map (uncurry $ formatToString ("Value: " % int % " " % int)) [(1,100), (2,350), ...]
map (\(x,y) -> formatToString ("Value: " % int % " " % int) x y) [(1,100), (2,350), ...]

It requires the GHC extension OverloadedStrings to function properly.

While formatToString ("Value: " % int % " " % int) has the type Int -> Int -> String, uncurrying it gives the type (Int, Int) -> String of which the input type matches the elements in the list.

The rewriting process can be broken down; assuming f = formatString ("Value: " ...),

map (\(x,y) -> f x y)  ≡  map (\(x,y) -> uncurry f (x,y))  ≡  map (uncurry f)

That is, first you uncurry f to achieve the function that accepts tuples, and then you perform a regular Eta-conversion since \(x,y) -> uncurry f (x,y) is equivalent to simply uncurry f. To print each line in the result, use mapM_:

mapM_ (putStrLn . uncurry $ formatToString ...) [(1,100), (2,350), ...]

If you run hlint YourFile.hs, these rewrites will be recommended to you.


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