std: Replace map_vec, map_vec2, iter_vec2 in std::result

Replace these with three functions based on iterators: collect, fold,
and fold_. The mapping part is replaced by iterator .map(), so the part
that these functions do is to accumulate the final Result<,> value.

* `result::collect` gathers `Iterator<Result<V, U>>` to `Result<~[V], U>`
* `result::fold` folds `Iterator<Result<T, E>>` to `Result<V, E>`
* `result::fold_` folds `Iterator<Result<T, E>>` to `Result<(), E>`

Author: blake2-ppc

src/librustc/middle/typeck/infer/combine.rs

@@ -68,7 +68,7 @@ use middle::typeck::infer::{InferCtxt, cres, ures};
 use middle::typeck::infer::{TypeTrace};
 use util::common::indent;
 
-use std::result::{iter_vec2, map_vec2};
+use std::result;
 use std::vec;
 use syntax::ast::{Onceness, purity};
 use syntax::ast;
@@ -275,9 +275,9 @@ pub fn super_tps<C:Combine>(
     // variance.
 
     if vec::same_length(as_, bs) {
-        iter_vec2(as_, bs, |a, b| {
-            eq_tys(this, *a, *b)
-        }).then(|| Ok(as_.to_owned()) )
+        result::fold_(as_.iter().zip(bs.iter())
+                      .map(|(a, b)| eq_tys(this, *a, *b)))
+            .then(|| Ok(as_.to_owned()))
     } else {
         Err(ty::terr_ty_param_size(
             expected_found(this, as_.len(), bs.len())))
@@ -427,7 +427,8 @@ pub fn super_fn_sigs<C:Combine>(
 {
     fn argvecs<C:Combine>(this: &C, a_args: &[ty::t], b_args: &[ty::t]) -> cres<~[ty::t]> {
         if vec::same_length(a_args, b_args) {
-            map_vec2(a_args, b_args, |a, b| this.args(*a, *b))
+            result::collect(a_args.iter().zip(b_args.iter())
+                            .map(|(a, b)| this.args(*a, *b)))
         } else {
             Err(ty::terr_arg_count)
         }
@@ -586,8 +587,9 @@ pub fn super_tys<C:Combine>(
 
       (&ty::ty_tup(ref as_), &ty::ty_tup(ref bs)) => {
         if as_.len() == bs.len() {
-            map_vec2(*as_, *bs, |a, b| this.tys(*a, *b) )
-                .chain(|ts| Ok(ty::mk_tup(tcx, ts)) )
+            result::collect(as_.iter().zip(bs.iter())
+                            .map(|(a, b)| this.tys(*a, *b)))
+                    .chain(|ts| Ok(ty::mk_tup(tcx, ts)) )
         } else {
             Err(ty::terr_tuple_size(
                 expected_found(this, as_.len(), bs.len())))

src/libstd/result.rs

@@ -18,8 +18,7 @@ use either;
 use iterator::Iterator;
 use option::{None, Option, Some, OptionIterator};
 use vec;
-use vec::{OwnedVector, ImmutableVector};
-use container::Container;
+use vec::OwnedVector;
 use to_str::ToStr;
 use str::StrSlice;
 
@@ -269,86 +268,76 @@ pub fn map_opt<T, U: ToStr, V>(o_t: &Option<T>,
     }
 }
 
-// FIXME: #8228 Replaceable by an external iterator?
-/// Maps each element in the vector `ts` using the operation `op`.  Should an
-/// error occur, no further mappings are performed and the error is returned.
-/// Should no error occur, a vector containing the result of each map is
-/// returned.
+/// Takes each element in the iterator: if it is an error, no further
+/// elements are taken, and the error is returned.
+/// Should no error occur, a vector containing the values of each Result
+/// is returned.
 ///
 /// Here is an example which increments every integer in a vector,
 /// checking for overflow:
 ///
-///     fn inc_conditionally(x: uint) -> result<uint,str> {
+///     fn inc_conditionally(x: uint) -> Result<uint, &'static str> {
 ///         if x == uint::max_value { return Err("overflow"); }
 ///         else { return Ok(x+1u); }
 ///     }
-///     map(~[1u, 2u, 3u], inc_conditionally).chain {|incd|
-///         assert!(incd == ~[2u, 3u, 4u]);
-///     }
+///     let v = [1u, 2, 3];
+///     let res = collect(v.iter().map(|&x| inc_conditionally(x)));
+///     assert!(res == Ok(~[2u, 3, 4]));
 #[inline]
-pub fn map_vec<T,U,V>(ts: &[T], op: &fn(&T) -> Result<V,U>)
-                      -> Result<~[V],U> {
-    let mut vs: ~[V] = vec::with_capacity(ts.len());
-    for t in ts.iter() {
-        match op(t) {
-          Ok(v) => vs.push(v),
-          Err(u) => return Err(u)
+pub fn collect<T, E, Iter: Iterator<Result<T, E>>>(mut iterator: Iter)
+    -> Result<~[T], E> {
+    let (lower, _) = iterator.size_hint();
+    let mut vs: ~[T] = vec::with_capacity(lower);
+    for t in iterator {
+        match t {
+            Ok(v) => vs.push(v),
+            Err(u) => return Err(u)
         }
     }
-    return Ok(vs);
+    Ok(vs)
 }
 
-// FIXME: #8228 Replaceable by an external iterator?
-/// Same as map, but it operates over two parallel vectors.
+/// Perform a fold operation over the result values from an iterator.
 ///
-/// A precondition is used here to ensure that the vectors are the same
-/// length.  While we do not often use preconditions in the standard
-/// library, a precondition is used here because result::t is generally
-/// used in 'careful' code contexts where it is both appropriate and easy
-/// to accommodate an error like the vectors being of different lengths.
+/// If an `Err` is encountered, it is immediately returned.
+/// Otherwise, the folded value is returned.
 #[inline]
-pub fn map_vec2<S, T, U: ToStr, V>(ss: &[S], ts: &[T],
-                                   op: &fn(&S,&T) -> Result<V,U>) -> Result<~[V],U> {
-    assert!(vec::same_length(ss, ts));
-    let n = ts.len();
-    let mut vs = vec::with_capacity(n);
-    let mut i = 0u;
-    while i < n {
-        match op(&ss[i],&ts[i]) {
-          Ok(v) => vs.push(v),
-          Err(u) => return Err(u)
+pub fn fold<T, V, E,
+            Iter: Iterator<Result<T, E>>>(
+            mut iterator: Iter,
+            mut init: V,
+            f: &fn(V, T) -> V)
+         -> Result<V, E> {
+    for t in iterator {
+        match t {
+            Ok(v) => init = f(init, v),
+            Err(u) => return Err(u)
         }
-        i += 1u;
     }
-    return Ok(vs);
+    Ok(init)
 }
 
-// FIXME: #8228 Replaceable by an external iterator?
-/// Applies op to the pairwise elements from `ss` and `ts`, aborting on
-/// error.  This could be implemented using `map_zip()` but it is more efficient
-/// on its own as no result vector is built.
+/// Perform a trivial fold operation over the result values
+/// from an iterator.
+///
+/// If an `Err` is encountered, it is immediately returned.
+/// Otherwise, a simple `Ok(())` is returned.
 #[inline]
-pub fn iter_vec2<S, T, U: ToStr>(ss: &[S], ts: &[T],
-                                 op: &fn(&S,&T) -> Result<(),U>) -> Result<(),U> {
-    assert!(vec::same_length(ss, ts));
-    let n = ts.len();
-    let mut i = 0u;
-    while i < n {
-        match op(&ss[i],&ts[i]) {
-          Ok(()) => (),
-          Err(u) => return Err(u)
-        }
-        i += 1u;
-    }
-    return Ok(());
+pub fn fold_<T, E, Iter: Iterator<Result<T, E>>>(
+             iterator: Iter)
+          -> Result<(), E> {
+    fold(iterator, (), |_, _| ())
 }
 
+
 #[cfg(test)]
 mod tests {
     use super::*;
 
     use either;
+    use iterator::range;
     use str::OwnedStr;
+    use vec::ImmutableVector;
 
     pub fn op1() -> Result<int, ~str> { Ok(666) }
 
@@ -431,4 +420,44 @@ mod tests {
         assert_eq!(r.to_either(), either::Right(100));
         assert_eq!(err.to_either(), either::Left(404));
     }
+
+    #[test]
+    fn test_collect() {
+        assert_eq!(collect(range(0, 0)
+                           .map(|_| Ok::<int, ()>(0))),
+                   Ok(~[]));
+        assert_eq!(collect(range(0, 3)
+                           .map(|x| Ok::<int, ()>(x))),
+                   Ok(~[0, 1, 2]));
+        assert_eq!(collect(range(0, 3)
+                           .map(|x| if x > 1 { Err(x) } else { Ok(x) })),
+                   Err(2));
+
+        // test that it does not take more elements than it needs
+        let functions = [|| Ok(()), || Err(1), || fail!()];
+
+        assert_eq!(collect(functions.iter().map(|f| (*f)())),
+                   Err(1));
+    }
+
+    #[test]
+    fn test_fold() {
+        assert_eq!(fold_(range(0, 0)
+                        .map(|_| Ok::<(), ()>(()))),
+                   Ok(()));
+        assert_eq!(fold(range(0, 3)
+                        .map(|x| Ok::<int, ()>(x)),
+                        0, |a, b| a + b),
+                   Ok(3));
+        assert_eq!(fold_(range(0, 3)
+                        .map(|x| if x > 1 { Err(x) } else { Ok(()) })),
+                   Err(2));
+
+        // test that it does not take more elements than it needs
+        let functions = [|| Ok(()), || Err(1), || fail!()];
+
+        assert_eq!(fold_(functions.iter()
+                        .map(|f| (*f)())),
+                   Err(1));
+    }
 }