Type Synonyms
Type synonyms are the simplest way to declare a new type as they simply provide an alias for one of the already existing types. For example, we already know that String is actually just a synonym for 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"]
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