> For the complete documentation index, see [llms.txt](https://docs.luaplusplus.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.luaplusplus.org/language-improvements/type-annotations/function-annotations.md).

# Function Annotations

Function annotations serve a similar purpose to type annotations, in that they tell the compiler what type of function to assign to this variable. This can be done either by assigning the type to a local variable or in the function itself.

```lua
local f: (number, string): boolean

-- Valid assignment
f = function(a: number, b: string): boolean
    return true
end
```

The code above demonstrates how to utilize function annotations correctly. Note that primitive types are interchangeable. If the types of functions are not identical to the definition, then the compiler will throw an exception. Below is another example of how to annotate a function with types:

```lua
local function f(a: number, b: number): boolean
    return a == b
end

-- Error: should return boolean not string
local function g(x: number): boolean
    return "Hello"
end
```


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