Functions and Lambdas

In Tomo, you can define functions with the func keyword:

func add(x:Int, y:Int -> Int)
    return x + y

Functions require you to explicitly write out the types of each argument and the return type of the function (unless the function doesn’t return any values).

For convenience, you can lump arguments with the same type together to avoid having to retype the same type name: func add(x, y:Int -> Int)

Default Arguments

Instead of giving a type, you can provide a default argument and the type checker will infer the type of the argument from that value:

func increment(x:Int, amount=1 -> Int)
    return x + amount

Default arguments are used to fill in arguments that were not provided at the callsite:

>> increment(5)
= 6

>> increment(5, 10)
= 15

Note: Default arguments are re-evaluated at the callsite for each function call, so if your default argument is func foo(x=random.int(1,10) -> Int), then each time you call the function without an x argument, it will give you a new random number.

Keyword Arguments

Tomo supports calling functions using keyword arguments to specify the values of any argument. Keyword arguments can be at any position in the function call and are bound to arguments first, followed by binding positional arguments to any unbound arguments, in order:

func foo(x:Int, y:Text, z:Num)
    return "x=$x y=$y z=$z"

>> foo(x=1, y="hi", z=2.5)
= "x=1 y=hi z=2.5"

>> foo(z=2.5, 1, "hi")
= "x=1 y=hi z=2.5"

As an implementation detail, all function calls are compiled to normal positional argument passing, the compiler just does the work to determine which order the arguments will be placed. Arguments are evaluated in the order in which they appear in code.

Function Caching

Tomo supports automatic function caching using the cached or cache_size=N attributes on a function definition:

func add(x, y:Int -> Int; cached)
    return x + y

Cached functions are outwardly identical to uncached functions, but internally, they maintain a table that maps a struct containing the input arguments to the return value for those arguments. The above example is functionally similar to the following code:

func _add(x, y:Int -> Int)
    return x + y

struct add_args(x,y:Int)
add_cache : @{add_args=Int} = @{}

func add(x, y:Int -> Int)
    args := add_args(x, y)
    if cached := add_cache[args]
        return cached
    ret := _add(x, y)
    add_cache[args] = ret
    return ret

You can also set a maximum cache size, which causes a random cache entry to be evicted if the cache has reached the maximum size and needs to insert a new entry:

func doop(x:Int, y:Text, z:[Int]; cache_size=100 -> Text)
    return "x=$x y=$y z=$z"

Inline Functions

Functions can also be given an inline attribute, which encourages the compiler to inline the function when possible:

func add(x, y:Int -> Int; inline)
    return x + y

This will directly translate to putting the inline keyword on the function in the transpiled C code.

Lambdas

In Tomo, you can define lambda functions, also known as anonymous functions, like this:

fn := func(x,y:Int): x + y

The normal form of a lambda is to give a return expression after the colon, but you can also use a block that includes statements:

fn := func(x,y:Int)
    if x == 0
        return y
    return x + y

Lambda functions must declare the types of their arguments, but do not require declaring the return type. Because lambdas cannot be recursive or corecursive (since they aren’t declared with a name), it is always possible to infer the return type without much difficulty. If you do choose to declare a return type, the compiler will attempt to promote return values to that type, or give a compiler error if the return value is not compatible with the declared return type.

Closures

When declaring a lambda function, any variables that are referenced from the enclosing scope will be implicitly copied into a heap-allocated userdata structure and attached to the lambda so that it can continue to reference those values. Captured values are copied to a new location at the moment the lambda is created and will not reflect changes to local variables.

func create_adder(n:Int -> func(i:Int -> Int))
    adder := func(i:Int)
        return n + i

    n = -1 // This does not affect the adder
    return adder
...
add10 := create_adder(10)
>> add10(5)
= 15

Under the hood, all user functions that are passed around in Tomo are passed as a struct with two members: a function pointer and a pointer to any captured values. When compiling the lambda to a function in C, we implicitly add a userdata parameter and access fields on that structure when we need to access variables from the closure. Captured variables can be modified by the lambda function, but those changes will only be visible to that particular lambda function.

Note: if a captured value is a pointer to a value that lives in heap memory, the pointer is copied, not the value in heap memory. This means that you can have a lambda that captures a reference to a mutable object on the heap and can modify that object. However, lambdas are not allowed to capture stack pointers and the compiler will give you an error if you attempt to do so.