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2012-02-15T15:39:58.857
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<blockquote> Editor's note: This answer is from a version of Rust prior to 1.0 and contains code that is not syntactically valid in Rust 1.0. </blockquote> If all you need is a way to define <code>apply</code>, try Rust's macro-by-example syntax extensions: <pre><code>fn main() { #macro[[#apply[f, [x, ...]], f(x, ...)]]; fn add(a: int, b: int) -> int { a + b } assert (#apply[add, [1, 15]] == 16); } </code></pre> The above code is from <a href="https://github.com/mozilla/rust/blob/master/src/test/run-pass/macro-by-example-1.rs" rel="nofollow noreferrer">the Rust test suite</a>. Sadly, the documentation on syntax extensions is a bit sparse at the moment. The <a href="http://doc.rust-lang.org/doc/rust.html#syntax-extension-expressions" rel="nofollow noreferrer">Rust reference manual</a> is probably your best bet--although the example it gives (of <code>apply</code>, no less!) is outdated, so I'm not sure how much of its information can be trusted. Update: <blockquote> All that's left is figuring out how to wrap add ... assert in a function with the right type signature that accepts arbitrary generators. </blockquote> I'm still not sure exactly how you're putting this all together, but here's a function that accepts any function that produces an <code>int</code>: <pre><code>use std; import std::rand; fn assert_even(num_gen: fn() -> int) -> (bool, int) { let num = num_gen(); ret (num % 2 == 0, num); } fn main() { let rng = rand::mk_rng(); let gen_even = {|| (rng.next() as int) * 2}; log(error, assert_even(gen_even)); } </code></pre> However, working with numbers in Rust is sort of a pain at the moment, and if you want to generalize <code>assert_even</code> to any numeric type you'll have to define interfaces/implementations and then declare <code>assert_even</code> with a bounded generic type: <pre><code>use std; import std::rand; iface is_even { fn is_even() -> bool; } impl of is_even for int { fn is_even() -> bool { self % 2 == 0 } } impl of is_even for u32 { fn is_even() -> bool { self % 2u == 0u } } fn assert_even<T: is_even>(num_gen: fn() -> T) -> (bool, T) { let num = num_gen(); ret (num.is_even(), num); } fn main() { let rng = rand::mk_rng(); let gen_even_int = {|| (rng.next() as int) * 2}; let gen_even_u32 = {|| rng.next() * 2u}; log(error, assert_even(gen_even_int)); log(error, assert_even(gen_even_u32)); } </code></pre> Side note: if you're interested in testing, you should check out Rust's typestate facilities. See <a href="http://doc.rust-lang.org/doc/rust.html#typestate-system" rel="nofollow noreferrer">the Rust manual here</a> for an example of what typestate does and the ways in which it is capable of enforcing program correctness. The way I understand it, it's basically a more powerful version of Eiffel's <a href="http://en.wikipedia.org/wiki/Design_by_contract" rel="nofollow noreferrer">design by contract</a>. Update 2: <blockquote> for_all accepts a single property (e.g. is_even or divisible_by) and a collection of generator functions. The generators are lambdas which return random values to pass as input to the property, e.g. [gen_int] for is_even or [gen_int, gen_int] for divisible_by. for_all will call the property using the generated values as a test case, printing +++ OK, passed 100 tests if the property returns true for 100 random test cases, or *** Failed! {test_case} if one of the test cases fails. </blockquote> This complete source file should fully demonstrate the behavior that you're looking for, hopefully (the definition of <code>for_all</code> is near the very bottom): <pre><code>use std; import std::rand; import std::io::println; iface is_even { fn is_even() -> bool; } impl of is_even for int { fn is_even() -> bool { self % 2 == 0 } } fn main() { let rng = rand::mk_rng(); // Cast to int here because u32 is lame let gen_even = {|| (rng.next() as int) * 2}; let gen_float = {|| rng.next_float()}; // Accepts generators that produce types that implement the is_even iface fn assert_even<T: is_even>(num_gen: fn() -> T) -> bool { let num = num_gen(); let prop_holds = num.is_even(); if !prop_holds { println(#fmt("Failure: %? is not even", num)); } ret prop_holds; } fn assert_divisible(num_gen1: fn() -> float, num_gen2: fn() -> float) -> bool { let dividend = num_gen1(), divisor = num_gen2(); let prop_holds = dividend / divisor == 0f; if !prop_holds { println(#fmt("Failure: %? are not divisible", (dividend, divisor))); } ret prop_holds; } // Begin anonymous closure here #macro[[#for_all[prop, [gen, ...]], {|| let passed_tests = 0; let prop_holds = true; // Nice iterators and break/continue are still being implemented, // so this loop is a bit crude. while passed_tests < 100 && prop_holds { prop_holds = prop(gen, ...); if prop_holds { passed_tests += 1; } } println(#fmt("Tests passed: %d", passed_tests)); ret 0; // Necessary to infer type of #for_all, might be a compiler bug }()]]; // Close anonymous closure and self-execute, then close #macro #for_all[assert_even, [gen_even]]; #for_all[assert_divisible, [gen_float, gen_float]]; } </code></pre> One more thing: the syntax extension mechanism is still fairly unpolished, so it's not possible to import macros from different crates. Until then, the definition of <code>#for_all</code> will have to appear in the file in which it is invoked.
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