list: use predicate to enforce non-empty requirement

src/comp/middle/last_use.rs

@@ -1,7 +1,7 @@
 import syntax::{visit, ast_util};
 import syntax::ast::*;
 import syntax::codemap::span;
-import std::list::{list, nil, cons, tail};
+import std::list::{is_not_empty, list, nil, cons, tail};
 import core::{vec, option};
 import std::list;
 
@@ -177,7 +177,9 @@ fn visit_block(tp: block_type, cx: ctx, visit: block()) {
     visit();
     local.second = true;
     visit();
-    cx.blocks = tail(cx.blocks);
+    let cx_blocks = cx.blocks;
+    check is_not_empty(cx_blocks);
+    cx.blocks = tail(cx_blocks);
     cx.current = join_branches(local.exits);
 }
 

src/comp/middle/resolve.rs

@@ -266,6 +266,7 @@ fn map_crate(e: @env, c: @ast::crate) {
             let imp = follow_import(*e, sc, *path, vi.span);
             if option::is_some(imp) {
                 let glob = {def: option::get(imp), item: vi};
+                check list::is_not_empty(sc);
                 alt list::head(sc) {
                   scope_item(i) {
                     e.mod_map.get(i.id).glob_imports += [glob];
@@ -455,6 +456,7 @@ fn visit_block_with_scope(b: ast::blk, sc: scopes, v: vt<scopes>) {
 }
 
 fn visit_decl_with_scope(d: @decl, sc: scopes, v: vt<scopes>) {
+    check list::is_not_empty(sc);
     let loc_pos = alt list::head(sc) {
       scope_block(_, _, pos) { pos }
       _ { @mutable 0u }

src/libstd/list.rs

@@ -97,6 +97,27 @@ fn has<copy T>(ls: list<T>, elt: T) -> bool {
     ret false;
 }
 
+/*
+Function: is_empty
+
+Returns true if the list is empty.
+*/
+pure fn is_empty<copy T>(ls: list<T>) -> bool {
+    alt ls {
+        nil. { true }
+        _ { false }
+    }
+}
+
+/*
+Function: is_not_empty
+
+Returns true if the list is not empty.
+*/
+pure fn is_not_empty<copy T>(ls: list<T>) -> bool {
+    ret !is_empty(ls);
+}
+
 /*
 Function: len
 
@@ -112,8 +133,11 @@ Function: tail
 
 Returns all but the first element of a list
 */
-pure fn tail<copy T>(ls: list<T>) -> list<T> {
-    alt ls { cons(_, tl) { ret *tl; } nil. { fail "list empty" } }
+pure fn tail<copy T>(ls: list<T>) : is_not_empty(ls) -> list<T> {
+    alt ls {
+        cons(_, tl) { ret *tl; }
+        nil. { fail "list empty" }
+    }
 }
 
 /*
@@ -121,8 +145,11 @@ Function: head
 
 Returns the first element of a list
 */
-pure fn head<copy T>(ls: list<T>) -> T {
-    alt ls { cons(hd, _) { ret hd; } nil. { fail "list empty" } }
+pure fn head<copy T>(ls: list<T>) : is_not_empty(ls) -> T {
+    alt ls {
+        cons(hd, _) { ret hd; }
+        nil. { fail "list empty" }
+    }
 }
 
 /*

src/test/run-pass/non-boolean-pure-fns.rs

@@ -14,7 +14,10 @@ pure fn nonempty_list<copy T>(ls: list<T>) -> bool { pure_length(ls) > 0u }
 // knowledge that ls is a cons node. Future work.
 // Also, this is pretty contrived since nonempty_list
 // could be a "tag refinement", if we implement those.
-fn safe_head<copy T>(ls: list<T>) : nonempty_list(ls) -> T { head(ls) }
+fn safe_head<copy T>(ls: list<T>) : nonempty_list(ls) -> T {
+    check is_not_empty(ls);
+    ret head(ls);
+}
 
 fn main() {
     let mylist = cons(@1u, @nil);

src/test/run-pass/unchecked-predicates.rs

@@ -1,4 +1,6 @@
 // Uses foldl to exhibit the unchecked block syntax.
+// TODO: since list's head/tail require the predicate "is_not_empty" now and
+// we have unit tests for list, this test might me not necessary anymore?
 use std;
 
 import std::list::*;
@@ -6,7 +8,10 @@ import std::list::*;
 // Can't easily be written as a "pure fn" because there's
 // no syntax for specifying that f is pure.
 fn pure_foldl<copy T, copy U>(ls: list<T>, u: U, f: block(T, U) -> U) -> U {
-    alt ls { nil. { u } cons(hd, tl) { f(hd, pure_foldl(*tl, f(hd, u), f)) } }
+    alt ls {
+        nil. { u }
+        cons(hd, tl) { f(hd, pure_foldl(*tl, f(hd, u), f)) }
+    }
 }
 
 // Shows how to use an "unchecked" block to call a general
@@ -22,7 +27,10 @@ pure fn nonempty_list<copy T>(ls: list<T>) -> bool { pure_length(ls) > 0u }
 // knowledge that ls is a cons node. Future work.
 // Also, this is pretty contrived since nonempty_list
 // could be a "tag refinement", if we implement those.
-fn safe_head<copy T>(ls: list<T>) : nonempty_list(ls) -> T { head(ls) }
+fn safe_head<copy T>(ls: list<T>) : nonempty_list(ls) -> T {
+    check is_not_empty(ls);
+    ret head(ls)
+}
 
 fn main() {
     let mylist = cons(@1u, @nil);

src/test/stdtest/list.rs

@@ -2,17 +2,23 @@ import core::*;
 
 use std;
 import std::list;
-import std::list::head;
-import std::list::tail;
-import std::list::from_vec;
+import std::list::{from_vec, head, is_not_empty, tail};
 import option;
 
 #[test]
 fn test_from_vec() {
     let l = from_vec([0, 1, 2]);
+
+    check is_not_empty(l);
     assert (head(l) == 0);
-    assert (head(tail(l)) == 1);
-    assert (head(tail(tail(l))) == 2);
+
+    let tail_l = tail(l);
+    check is_not_empty(tail_l);
+    assert (head(tail_l) == 1);
+
+    let tail_tail_l = tail(tail_l);
+    check is_not_empty(tail_tail_l);
+    assert (head(tail_tail_l) == 2);
 }
 
 #[test]
@@ -24,9 +30,17 @@ fn test_from_vec_empty() {
 #[test]
 fn test_from_vec_mut() {
     let l = from_vec([mutable 0, 1, 2]);
+
+    check is_not_empty(l);
     assert (head(l) == 0);
-    assert (head(tail(l)) == 1);
-    assert (head(tail(tail(l))) == 2);
+
+    let tail_l = tail(l);
+    check is_not_empty(tail_l);
+    assert (head(tail_l) == 1);
+
+    let tail_tail_l = tail(tail_l);
+    check is_not_empty(tail_tail_l);
+    assert (head(tail_tail_l) == 2);
 }
 
 #[test]