Introducing Pairs in Haskell

Look … here is a pair. No, not a PEAR pear but a pair PAIR, as in two items, two related items of fruit, an apple and a banana. Ordering is important in pairs. First I will eat the apple,

and then, after that, I’ll eat my banana So, two related items with a set ordering. Now let’s think about pairs in Haskell.

In Haskell syntax, you express a pair like this:

two parentheses, a comma in between the two elements so for instance ( 1 , 2 ) is an Integer pair. it’s a composite value with two individual items inside. Haskell has built-in accessor functions for pairs as well. If I say first (spelled fst) 1 comma 2 then that will return the first element from this pair which is 1 and if I say second (spelled) snd 1 2 then that will return 2 which is the second element in this composite value here.

Pairs don’t necessarily have to have elements of the same type inside them, so for instance, I might say something like 5 (which is an Integer) comma, string quotes gold rings and there the first element is an Integer, the second element is a String or character list, as you know. So this pair has two elements of different types inside it. I can be more general still, so it might be that I decide to have more than two elements, I could have a triple, and then I would have three individual elements, with commas separating them, and the brackets outside. Or a quadruple, or a quintuple. In the general case we call the thing a tuple which has n items inside it.

Pairs are not to be confused with lists. You have already seen lists in some previous example videos and sessions. So what I would like you to do is to think about concrete differences between pairs and lists , and maybe comment on those differences in the comments section below. But for now, keep on coding!