29 lines
891 B
Rust
29 lines
891 B
Rust
use std;
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import std::list::*;
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pure fn pure_length_go<T: copy>(ls: list<T>, acc: uint) -> uint {
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alt ls { nil { acc } cons(_, tl) { pure_length_go(*tl, acc + 1u) } }
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}
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pure fn pure_length<T: copy>(ls: list<T>) -> uint { pure_length_go(ls, 0u) }
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pure fn nonempty_list<T: copy>(ls: list<T>) -> bool { pure_length(ls) > 0u }
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// Of course, the compiler can't take advantage of the
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// knowledge that ls is a cons node. Future work.
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// Also, this is pretty contrived since nonempty_list
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// could be a "enum refinement", if we implement those.
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fn safe_head<T: copy>(ls: list<T>) : nonempty_list(ls) -> T {
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check is_not_empty(ls);
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ret head(ls);
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}
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fn main() {
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let mylist = cons(@1u, @nil);
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// Again, a way to eliminate such "obvious" checks seems
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// desirable. (Tags could have postconditions.)
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check (nonempty_list(mylist));
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assert (*safe_head(mylist) == 1u);
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}
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