[SOLVED] Why is the function's name repeated when defining the function via pattern matching?

Why is the function's name repeated when defining the function via pattern matching?

From Miran Lipovača's Learn you a Haskell for great good!:

lucky :: (Integral a) => a -> String  
lucky 7 = "LUCKY NUMBER SEVEN!"  
lucky x = "Sorry, you're out of luck, pal!"

In this definition of function lucky using pattern matching, why is the function's name repeated? When should I not be repeating the function's name? What is the meaning of it?

Solution

What you are seeing is pattern match in action.

I will show you another example:

test 1 = "one"
test 2 = "two"
test 3 = "three"

Demo in ghci:

ghci> test 1
"one"
ghci> test 2
"two"
ghci> test 3
"three"
ghci> test 4
"*** Exception: Non-exhaustive patterns in function test

So, when you call any function, the runtime system will try to match the input with the defined function. So a call to test 3 will initially check test 1 and since 1 is not equal to 3, it will move on to the next definition. Again since 2 is not equal to 3, it will move to the next defintion. In the next definiton since 3 is equal to 3 it will return "three" String back. When you try to pattern match something, which doesn't exist at all, the program throws the exception.