Functions
Functions make data flow explicit with parameter and return types. Purity is optional: func declares an impure function; pure func adds a compiler-enforced purity contract.
Parameters
Parameters are written as name: Type inside parentheses. In add(a: Int, b: Int), the two arguments are bound to a and b inside the function.
func add(a: Int, b: Int) -> Int:
a + b
func main(args: List[String]) -> Void:
print(add(2, 3).to_string()) -- prints: 5
Return Types
The return type follows ->. The last expression supplies the returned value. For named functions, omitting -> means Void, not an inferred result type; Void means no returned value.
pure func label(score: Int) -> String:
"score: " + score.to_string()
Pure and Impure Functions
Impure functions may perform I/O and call either pure or impure functions. Pure functions cannot call impure functions. The Pure Functions page explains the restricted exception for debug diagnostics.
pure func square(value: Int) -> Int:
value * value
func main(args: List[String]) -> Void:
print(square(4).to_string()) -- prints: 16
Functions as Values
Functions are values: bind a named function or pass it as an argument. A callback type such as (Int) -> Int specifies its input and output; pure (Int) -> Int also requires purity.
func increment(value: Int) -> Int:
value + 1
func apply(value: Int, transform: (Int) -> Int) -> Int:
transform(value)
func main(args: List[String]) -> Void:
next: (Int) -> Int = increment
print(next(10).to_string()) -- prints: 11
print(apply(10, next).to_string()) -- prints: 11
Anonymous Functions
func(value): expression defines a callback inline. Its parameter types can come from the expected function type. pure func(...) explicitly declares a pure callback; an expected pure type can also infer purity when the body has no impure operations.
func main(args: List[String]) -> Void:
increment = func(value: Int) -> Int: value + 1
print(increment(4).to_string()) -- prints: 5
add: (Int, Int) -> Int = func(left, right): left + right
print(add(2, 4).to_string()) -- prints: 6
double: pure (Int) -> Int = pure func(value: Int): value * 2
print(double(4).to_string()) -- prints: 8
-- The expected pure function type also permits purity inference.
triple: pure (Int) -> Int = func(value): value * 3
print(triple(3).to_string()) -- prints: 9
Local Helpers
Use a small helper when the public function wants a simpler interface than the implementation.
pure func outer(n: Int) -> Int:
pure func inner(x: Int) -> Int:
x + 1
inner(n)
Recursion
Recursive functions call themselves directly.
pure func count_down(n: Int) -> Int:
if n == 0:
0
else:
count_down(n - 1)
@tail_recursive
@tail_recursive asks the compiler to verify the recursive call is in tail position.
@tail_recursive
pure func loop(n: Int, acc: Int) -> Int:
if n <= 0:
acc
else:
loop(n - 1, acc + n)
Example
func factorial(n: Int) -> Int:
if n <= 1:
1
else:
n * factorial(n - 1)
@tail_recursive
func factorial_loop(n: Int, acc: Int) -> Int:
if n <= 1:
acc
else:
factorial_loop(n - 1, n * acc)
func main(args: List[String]) -> Void:
print(factorial(5)) -- prints: 120
print(factorial_loop(5, 1)) -- prints: 120
Functions and Callbacks
func add(left: Int, right: Int) -> Int:
left + right
pure func square(value: Int) -> Int:
value * value
func increment(value: Int) -> Int:
value + 1
func apply(value: Int, transform: (Int) -> Int) -> Int:
transform(value)
func main(args: List[String]) -> Void:
print(add(2, 3).to_string()) -- prints: 5
print(square(4).to_string()) -- prints: 16
next: (Int) -> Int = increment
print(apply(10, next).to_string()) -- prints: 11
print(apply(10, func(value): value + 2).to_string()) -- prints: 12
Try it
blorp run factorial.brp