Real World Haskell

Important Built-in Typeclasses

Now that we’ve discussed defining your own typeclasses and making your types instances of typeclasses, it’s time to introduce you to typeclasses that are a standard part of the Haskell Prelude. As we mentioned at the beginning of this chapter, typeclasses are at the core of some important aspects of the language. We’ll cover the most common ones here. For more details, the Haskell library reference is a good resource. It will give you a description of the typeclasses and usually also will tell you which functions you must implement to have a complete definition.

Show

The Show typeclass is used to convert values to Strings. It is perhaps most commonly used to convert numbers to Strings, but it is defined for so many types that it can be used to convert quite a bit more. If you have defined your own types, making them instances of Show will make it easy to display them in ghci or print them out in programs.

The most important function of Show is show. It takes one argument—the data to convert. It returns a String representing that data. ghci reports the type of show like this:

ghci> :type show
show :: (Show a) => a -> String

Let’s look at some examples of converting values to strings:

ghci> show 1
"1"
ghci> show [1, 2, 3]
"[1,2,3]"
ghci> show (1, 2)
"(1,2)"

Remember that ghci displays results as they would be entered into a Haskell program. So the expression show 1 returns a single-character string containing the digit 1. That is, the quotes are not part of ...