7. Write a code (and explain your code) in ML Programming Language

Define a function first_list : (a * b) list -> a list which takes in input a list of pairs and gives back the list consisting of the first elements only, Examples:

first_list [ ] = [ ]

first_list [ (1,2), (1,3) ] = [ 1,1]

first_list [ (1, a), (2,b), (3, c) ] = [ 1,2,3 ]

first_list [ ( [ ] , a) , ( [ 1 ] , b) , ( [ 1,2 ] , c) ] = [ [ ] , [ 1] , [ 1,2 ] ]