Contracts

Contracts are optional pre- and post-conditions checks that the compiler may use for optimization and runtime checks. Note that compilers are not obliged to process pre- and post-conditions at all. However, violating either pre- or post-conditions is considered undefined behaviour, so a compiler may optimize as if they always hold – even if a potential bug may cause them to be violated.

Pre-conditions

Pre-conditions are usually used to validate incoming arguments. Each condition must be an expression that can be evaluated to a boolean. Pre-conditions use the @require annotation, and optionally can have an error message to display after them.

<*
 @require foo > 0, foo < 1000 : "optional error msg"
*>
fn int testFoo(int foo)
{
    return foo * 10;
}

Post conditions

Post conditions are evaluated to make checks on the resulting state after passing through the function. The post condition uses the @ensure annotation. Where return is used to represent the return value from the function.

<*
 @require foo != null
 @ensure return > foo.x
*>
fn uint checkFoo(Foo* foo)
{
    uint y = abs(foo.x) + 1;
    // If we had row: foo.x = 0, then this would be a compile time error.
    return y * abs(foo.x);
}

Parameter annotations

@param supports [in] [out] and [inout]. These are only applicable for pointer arguments. [in] disallows writing to the variable, [out] disallows reading from the variable. Without an annotation, pointers may both be read from and written to without checks. If an & is placed in front of the annotation (e.g. [&in]), then this means the pointer must be non-null and is checked for null.

Type	readable?	writable?	use as “in”?	use as “out”?	use as “inout”?
no annotation	Yes	Yes	Yes	Yes	Yes
in	Yes	No	Yes	No	No
out	No	Yes	No	Yes	No
inout	Yes	Yes	Yes	Yes	Yes

However, it should be noted that the compiler might not detect whether the annotation is correct or not! This program might compile, but will behave strangely:

fn void bad_func(int* i)
{
    *i = 2;
}

<*
 @param [&in] i
*>
fn void lying_func(int* i)
{
    bad_func(i); // The compiler might not check this!
}

fn void test()
{
    int a = 1;
    lying_func(&a);
    io::printf("%d", a); // Might print 1!
}

However, compilers will usually detect this:

<*
 @param [&in] i
*>
fn void bad_func(int* i)
{
    *i = 2; // <- Compiler error: cannot write to "in" parameter
}

Pure in detail

The pure annotation allows a program to make assumptions in regard to how the function treats global variables. Unlike for const, a pure function is not allowed to call a function which is known to be impure.

However, just like for const the compiler might not detect whether the annotation is correct or not! This program might compile, but will behave strangely:

int i = 0;

alias SecretFn = fn void();

fn void bad_func()
{
    i = 2;
}

<*
 @pure
*>
fn void lying_func(SecretFn f)
{
    f(); // The compiler cannot reason about this!
}

fn void main()
{
    i = 1;
    lying_func(&bad_func);
    io::printf("%d", i); // Might print 1!
}

However, compilers will usually detect this:

int i = 0;

alias SecretFn = fn void();

fn void bad_func()
{
    i = 2;
}

<*
 @pure
*>
fn void lying_func(SecretFn f)
{
    f(); // <- ERROR: Only '@pure' functions may be called.
}

fn void main()
{
    i = 1;
    lying_func(&bad_func);
    io::printf("%d", i); // Might print 1!
}

Consequently, circumventing “pure” annotations is undefined behaviour.

Pre-conditions for macros

In order to check macros, it’s often useful to use the builtin $defined function which returns true if the code inside would pass semantic checking.

<*
 @require $defined(resource.open, resource.open()), `Expected resource to have an "open" function`
 @require resource != nil
 @require $assignable(resource.open(), void*)
*>
macro open_resource(resource)
{
    return resource.open();
}