216 lines
5.2 KiB
Rust
216 lines
5.2 KiB
Rust
#[doc = "A type representing either success or failure"];
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import either::either;
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#[doc = "The result type"]
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enum result<T, U> {
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#[doc = "Contains the successful result value"]
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ok(T),
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#[doc = "Contains the error value"]
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err(U)
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}
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#[doc = "
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Get the value out of a successful result
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# Failure
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If the result is an error
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"]
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pure fn get<T: copy, U>(res: result<T, U>) -> T {
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alt res {
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ok(t) { t }
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err(the_err) {
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// FIXME: have a run-fail test for this
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unchecked{ fail #fmt("get called on error result: %?", the_err); }
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}
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}
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}
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#[doc = "
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Get the value out of an error result
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# Failure
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If the result is not an error
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"]
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pure fn get_err<T, U: copy>(res: result<T, U>) -> U {
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alt res {
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err(u) { u }
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ok(_) {
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fail "get_error called on ok result";
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}
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}
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}
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#[doc = "Returns true if the result is `ok`"]
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pure fn success<T, U>(res: result<T, U>) -> bool {
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alt res {
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ok(_) { true }
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err(_) { false }
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}
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}
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#[doc = "Returns true if the result is `error`"]
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pure fn failure<T, U>(res: result<T, U>) -> bool {
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!success(res)
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}
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#[doc = "
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Convert to the `either` type
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`ok` result variants are converted to `either::right` variants, `err`
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result variants are converted to `either::left`.
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"]
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pure fn to_either<T: copy, U: copy>(res: result<U, T>) -> either<T, U> {
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alt res {
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ok(res) { either::right(res) }
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err(fail_) { either::left(fail_) }
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}
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}
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#[doc = "
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Call a function based on a previous result
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If `res` is `ok` then the value is extracted and passed to `op` whereupon
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`op`s result is returned. if `res` is `err` then it is immediately returned.
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This function can be used to compose the results of two functions.
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Example:
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let res = chain(read_file(file)) { |buf|
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ok(parse_buf(buf))
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}
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"]
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fn chain<T, U: copy, V: copy>(res: result<T, V>, op: fn(T) -> result<U, V>)
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-> result<U, V> {
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alt res {
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ok(t) { op(t) }
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err(e) { err(e) }
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}
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}
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#[doc = "
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Call a function based on a previous result
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If `res` is `err` then the value is extracted and passed to `op`
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whereupon `op`s result is returned. if `res` is `ok` then it is
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immediately returned. This function can be used to pass through a
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successful result while handling an error.
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"]
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fn chain_err<T: copy, U: copy, V: copy>(
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res: result<T, V>,
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op: fn(V) -> result<T, U>)
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-> result<T, U> {
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alt res {
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ok(t) { ok(t) }
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err(v) { op(v) }
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}
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}
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impl extensions<T:copy, E:copy> for result<T,E> {
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fn chain<U:copy>(op: fn(T) -> result<U,E>) -> result<U,E> {
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chain(self, op)
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}
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fn chain_err<F:copy>(op: fn(E) -> result<T,F>) -> result<T,F> {
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chain_err(self, op)
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}
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}
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#[doc = "
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Maps each element in the vector `ts` using the operation `op`. Should an
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error occur, no further mappings are performed and the error is returned.
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Should no error occur, a vector containing the result of each map is
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returned.
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Here is an example which increments every integer in a vector,
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checking for overflow:
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fn inc_conditionally(x: uint) -> result<uint,str> {
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if x == uint::max_value { ret err(\"overflow\"); }
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else { ret ok(x+1u); }
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}
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map([1u, 2u, 3u], inc_conditionally).chain {|incd|
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assert incd == [2u, 3u, 4u];
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}
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"]
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fn map<T,U:copy,V:copy>(ts: [T], op: fn(T) -> result<V,U>) -> result<[V],U> {
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let mut vs: [V] = [];
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vec::reserve(vs, vec::len(ts));
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for vec::each(ts) {|t|
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alt op(t) {
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ok(v) { vs += [v]; }
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err(u) { ret err(u); }
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}
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}
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ret ok(vs);
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}
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#[doc = "Same as map, but it operates over two parallel vectors.
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A precondition is used here to ensure that the vectors are the same
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length. While we do not often use preconditions in the standard
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library, a precondition is used here because result::t is generally
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used in 'careful' code contexts where it is both appropriate and easy
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to accommodate an error like the vectors being of different lengths."]
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fn map2<S,T,U:copy,V:copy>(ss: [S], ts: [T], op: fn(S,T) -> result<V,U>)
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: vec::same_length(ss, ts) -> result<[V],U> {
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let n = vec::len(ts);
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let mut vs = [];
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vec::reserve(vs, n);
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let mut i = 0u;
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while i < n {
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alt op(ss[i],ts[i]) {
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ok(v) { vs += [v]; }
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err(u) { ret err(u); }
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}
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i += 1u;
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}
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ret ok(vs);
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}
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#[doc = "
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Applies op to the pairwise elements from `ss` and `ts`, aborting on
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error. This could be implemented using `map2()` but it is more efficient
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on its own as no result vector is built.
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"]
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fn iter2<S,T,U:copy>(ss: [S], ts: [T],
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op: fn(S,T) -> result<(),U>)
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: vec::same_length(ss, ts)
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-> result<(),U> {
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let n = vec::len(ts);
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let mut i = 0u;
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while i < n {
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alt op(ss[i],ts[i]) {
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ok(()) { }
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err(u) { ret err(u); }
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}
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i += 1u;
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}
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ret ok(());
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}
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#[cfg(test)]
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mod tests {
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fn op1() -> result::result<int, str> { result::ok(666) }
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fn op2(&&i: int) -> result::result<uint, str> {
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result::ok(i as uint + 1u)
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}
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fn op3() -> result::result<int, str> { result::err("sadface") }
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#[test]
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fn chain_success() {
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assert get(chain(op1(), op2)) == 667u;
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}
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#[test]
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fn chain_failure() {
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assert get_err(chain(op3(), op2)) == "sadface";
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}
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}
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