Note: All of this capability is implemented except overloaded functions, throws, pure functions.

All processing logic (including expressions and blocks) for programs are specified inside functions. We have already shown how to call functions as part of an expression. Now, let's show how to declare and implement functions?

Let's start with a simple example of a function definition and its use:

// Define the 'square' function
fn square(a f32) f32:
	return a*a

// Use square to calculate the area of a circle	
area = Float.pi * square(r)

The function definition for 'square' specifies its:

Notice how the function call passes the value 'r' to the 'square' function. This value is caught by the function as the floating-point parameter 'a'. The function body multiplies 'a' to itself, and then returns the calculated floating-point value to the caller.

Function name

The function name is typically simple: the unique name for the function, used when calling the function. If a function name requires non-alphanumeric characters, enclose the name in back ticks (for example, `+` is a valid name for an operator function which adds values).

Overloaded functions and their names

It can sometimes be convenient to define more than one function with the same name. This is called function overloading. It typically occurs when all same-named functions perform essentially the same semantic operation, but do so using different number or types of parameter values. The compiler disambiguates which particular function to use based on the number and types of the caller's arguments.

Even when overloaded functions share the same name, it can be useful to give each overload a distinct name. Having a distinct name can make public interfaces more durable, debugging easier, and make it possible to explicitly designate which overload to obtain a reference to (e.g., by '&').

To specify both a shared and distinct name for an overloaded function, choose one of these two syntax options: concatenation via '+' (when the distinct name begins with the shared name) or different names separated by '|'.

// The shared name is 'add'.  The distinct name is 'addfloat' (concatenating add with float)
fn add+float(a Comp, b f32) Comp:

// The shared name is 'add'. The distinct name is 'compfloatadder'
fn add|compdynadder(a Comp, b dyn) Comp:

A caller may refer to an overloaded function by its shared or distinct name.

Parameter Variables

The declaration of a functions's parameters is a comma-separated list of typed variables (name and type), enclosed in parentheses. Each parameter variable corresponds on a one-to-one basis to the value arguments passed to the function when it is called. They must match in both number and type:

// Define a function that returns the addition of its two parameters
fn add(a i32, b i32) i32:
	return a+b

add(2,3)  // returns 5, since a is set to 2 and b to 3
add(2)    // ERROR! add() needs exactly two values, not one or three
add(2.0, 4.0)  // ERROR! add() needs integers, not floating point numbers

Parameters are local variables, usable within the function. Each parameter declaration in the function's signature mimics a local variable declaration, even allowing specification of mutability and a default value.

Parameter Mutability

When no permission is specified before the parameter name (as in the example above), imm is assumed. This forbids the function's logic from changing the value of the passed parameter.

To allow a parameter's value to be changed within the function's logic, precede the name with mut. Any change to the parameter's value in the function logic will have no effect on the caller, since the parameter's value is a copy of the caller's passed value.

fn mutator(a i32, mut b i32):
	a = 34    // ERROR! a is immutable and may not be changed
	b = 4     // b may be changed since it is declared 'mut'

number = 3
mutator(5, number)
// number is still 3, despite mutator() changing b to 4

Parameter Defaults

If desired, default values may be specified for any parameter value using the assignment operator '='. The default value is what we want the parameter to have if the caller provides no value for it. The default value can only be a constant value.

fn next(nbr i32, incr i32 = 1) i32:
	return nbr + incr

next(5,2) // returns 7
next(4)   // returns 5 (using incr's default value of 1)
next()    // ERROR! no default value for nbr

Return types

This comma-separated list specifies the type(s) of all values returned by the function:

// A function that returns a single integer value
fn getint() i32:

// A function that returns both an integer and float tuple value 
fn gettuple() i32, f32:

// A function that returns no values (equivalent to returning 'void'):
fn get():

throws may be specified after the return types to show that a function may throw an error value (using throw instead of return) if something goes wrong. See Result for more information.

// returns an integer on success, or else throws ErrorCond value for an exception
fn dosomething() i32 throws ErrorCond

Implementation Block

The function's implementation block is like any regular block, consisting of a ordered sequence of statements that perform the function's logic.

Local variables

All local variables declared within the function's block (including the function's parameters) provide a temporary working state for the function. Every time a function is called, space is allocated on the execution stack for all its declared local variables. When the function is finished and returns to its caller, its local variable space is automatically freed from the stack.

fn summult(a f32, b f32) f32:
	imm sum = a + b    // local variable declaration
	imm mult = a * b   // local variable declaration
	return sum / mult

This means that local variables are exclusive to that call. Two identically-named local variables in different functions, or even two calls to the same function, will not collide with each other.

Return Statement

A return statement may be placed at the end of any block. When encountered, execution of the function ceases and the comma-separated values specified after return are returned to the caller. The number and types of all return values must match the return types declared on the function's signature.

fn ceil(x i32) i32, i32:
	if x > 6:
		return x, 6
	return x, x

a,b = ceil(8)       // returns 8,6
a,b = ceil(3)       // returns 3,3

Implicit returns

A function does not have to specify a return statement at the end of its main block. If the function signature does not declare a return value, the function just returns after the last statement is performed.

If the function signature declares that values must be returned, an attempt is made to matching the values on the last statement or block of the function:

Should the correct number of type-matching return values not be found as described above, a compile error will result.

Recursive Functions

A function may call itself recursively:

fn factorial(x i32, prod i32 = 1):
  if x<=1:
    return prod 
  factorial(x-1, prod*x)

If the function returns a single value calculated by any function, this may be "tail-call" optimized. This optimization improves performance and reduces the risk of execution stack overrun with recursive calls.

Pure functions

Sometimes it is valuable when a function guarantees that its only work involves calculating return value(s) derived wholly from the information passed as parameters. Such functions are called pure. Pure functions may not:

Functions complying with these constraints should be marked as pure.

pure fn factorial(mut n i32) i32:
  if n<=1:
    return 1 
  mut result = 1
  while n-- > 1:
    result *= n

A key benefit for marking functions pure: it makes code easier to understand, debug and test. When examining logic that uses a pure function, we can be confident that:

Pure functions are also beneficial due to the safety guarantees they offer when:

Strong purity

The compiler only enforces the purity constraints listed above, resulting in what some may consider a weak form of purity. A stronger form of purity (referential transparency) may be obtained by ensuring:

Only a strongly pure functions can be safely memo-ized.

Inline functions

A function can be marked as "inline". For example:

fn incr(x i32) inline:

Any time another piece of code calls an inlined function or method, the called function's logic is actually generated within the caller's logic. For small inlined functions, this can offer a significant performance advantage, not only because it eliminates the overhead of call/return, but more importantly because it is very instruction-cache friendly.

Note: It is impossible to borrow a reference to an inlined function.