> For the complete documentation index, see [llms.txt](https://haskell.hpmeducation.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://haskell.hpmeducation.com/cutom-types/declaring-types/type-synonyms.md).

# 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:

```haskell
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:

```haskell
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:

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

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


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