Type Synonyms

Type synonyms are the simplest way to declare a new type as they simply provide an alias for an already existing type. For example, we already know that String is actually just a synonym for a list of Chars, and it is defined as:

type String = [Char]

We can use the declared type synonyms to define other types as well. We can define a type for a list of Strings:

type StringList = [String]

It is important to note that the type synonyms and their base types are interchangeable in almost all cases. That means that any function that has a type signature including a list of strings ([String]) could be used on an element that has the type of StringList as they are just synonyms:

reverseStringList :: StringList -> StringList
reverseStringList xs = reverse xs
-- interchangeable types StringList | [String]
reverseStringList :: [String] -> [String]
reverseStringList xs = reverse xs

ghci> reverseStringList ["abc", "123"]
["123","abc"]