Extract the core of parser macros into a separate crate

1 files changed, 190 insertions, 0 deletions

diff --git a/iter_patterns/src/lib.rs b/iter_patterns/src/lib.rs
new file mode 100644
index 0000000..75fe51c
--- /dev/null
+++ b/iter_patterns/src/lib.rs
@@ -0,0 +1,190 @@
+#![feature(slice_patterns)]
+
+/* Destructure an iterator using the syntax of slice_patterns.
+ * Wraps the match body in `Some` if there was a match; returns
+ * `None` otherwise.
+ * Contrary to slice_patterns, this allows moving out
+ * of the iterator.
+ * A variable length pattern (`x..`) is only allowed as the last
+ * pattern, unless the iterator is double-ended.
+ *
+ * Example:
+ * ```
+ * let vec = vec![Some(1), Some(2), None];
+ *
+ * destructure_iter!(vec.into_iter();
+ *     [Some(x), y.., z] => {
+ *         // x: usize
+ *         // y: impl Iterator<Option<usize>>
+ *         // z: Option<usize>
+ *     }
+ * )
+ * ```
+ *
+*/
+#[macro_export]
+macro_rules! destructure_iter {
+    // Variable length pattern
+    (@match 0, $iter:expr, ($body:expr), $x:ident.., $($rest:tt)*) => {
+        $crate::destructure_iter!(@match 1, $iter, ({
+            let $x = $iter;
+            $body
+        }), $($rest)*)
+    };
+    // Variable length pattern without a binder
+    (@match 0, $iter:expr, ($body:expr), .., $($rest:tt)*) => {
+        $crate::destructure_iter!(@match 1, $iter, ($body), $($rest)*)
+    };
+    // Single item pattern
+    (@match 0, $iter:expr, ($body:expr), $x:pat, $($rest:tt)*) => {
+        if let Some($x) = $iter.next() {
+            $crate::destructure_iter!(@match 0, $iter, ($body), $($rest)*)
+        } else {
+            None
+        }
+    };
+    // Single item pattern after a variable length one: declare reversed and take from the end
+    (@match 1, $iter:expr, ($body:expr), $x:pat, $($rest:tt)*) => {
+        $crate::destructure_iter!(@match 1, $iter, (
+            if let Some($x) = $iter.next_back() {
+                $body
+            } else {
+                None
+            }
+        ), $($rest)*)
+    };
+
+    // Check no elements remain
+    (@match 0, $iter:expr, ($body:expr) $(,)*) => {
+        if $iter.next().is_some() {
+            None
+        } else {
+            $body
+        }
+    };
+    // After a variable length pattern, everything has already been consumed
+    (@match 1, $iter:expr, ($body:expr) $(,)*) => {
+        $body
+    };
+
+    ($iter:expr; [$($args:tt)*] => $body:expr) => {
+        {
+            #[allow(unused_mut)]
+            let mut iter = $iter;
+            $crate::destructure_iter!(@match 0, iter, (Some($body)), $($args)*,)
+        }
+    };
+}
+
+
+/* Pattern-match on a vec using the syntax of slice_patterns.
+ * Wraps the match body in `Some` if there was a match; returns
+ * `None` otherwise.
+ * A variable length pattern (`x..`) returns an iterator.
+ *
+ * Example:
+ * ```
+ * let vec = vec![Some(1), Some(2), None];
+ *
+ * match_vec!(vec;
+ *     [Some(x), y.., z] => {
+ *         // x: usize
+ *         // y: impl Iterator<Option<usize>>
+ *         // z: Option<usize>
+ *     }
+ *     [x, Some(0)] => {
+ *         // x: Option<usize>
+ *     },
+ *     [..] => { }
+ * )
+ * ```
+ *
+*/
+#[macro_export]
+macro_rules! match_vec {
+    ($arg:expr; $( [$($args:tt)*] => $body:expr ),* $(,)*) => {
+        {
+            let vec = $arg;
+            // Match as references to decide which branch to take
+            // I think `match_default_bindings` should make this always work but
+            // there may be some patterns this doesn't capture.
+            #[allow(unused_variables, unreachable_patterns)]
+            match vec.as_slice() {
+                $(
+                    [$($args)*] => {
+                        // Actually consume the values
+                        #[allow(unused_mut)]
+                        let mut iter = vec.into_iter();
+                        $crate::destructure_iter!(iter; [$($args)*] => $body)
+                    }
+                )*
+                _ => None,
+            }
+        }
+    };
+}
+
+
+/* Pattern-match on an iterator using the syntax of slice_patterns.
+ * Wraps the match body in `Some` if there was a match; returns
+ * `None` otherwise.
+ *
+ * Example:
+ * ```
+ * let vec = vec![Some(1), Some(2), None];
+ *
+ * match_iter!(vec.into_iter();
+ *     [Some(x), y.., z] => {
+ *         // x: usize
+ *         // y: impl Iterator<Option<usize>>
+ *         // z: Option<usize>
+ *     }
+ *     [x, Some(0)] => {
+ *         // x: Option<usize>
+ *     },
+ *     [..] => {
+ * )
+ * ```
+ *
+*/
+#[macro_export]
+macro_rules! match_iter {
+    ($arg:expr; $($args:tt)*) => {
+        {
+            let vec: Vec<_> = $arg.collect();
+            $crate::match_vec!(vec, $($args)*)
+        }
+    };
+}
+
+
+#[test]
+fn test() {
+    let test = |v: Vec<Option<isize>>| {
+        match_vec!(v.into_iter();
+            [Some(_x), None, None] => 4,
+            [Some(_x), None] => 2,
+            [None, Some(y)] => 1,
+            [None, _y..] => 3,
+            [_x.., Some(y), Some(z)] => y - z,
+            [] => 0,
+            [..] => -1,
+        )
+        .unwrap()
+    };
+
+    assert_eq!(test(vec![Some(0), None, None]), 4);
+    assert_eq!(test(vec![Some(0), None]), 2);
+    assert_eq!(test(vec![None, Some(0)]), 1);
+    assert_eq!(test(vec![Some(1), Some(2), Some(5), Some(14)]), -9);
+    assert_eq!(test(vec![None]), 3);
+    assert_eq!(test(vec![]), 0);
+    assert_eq!(test(vec![Some(0)]), -1);
+
+    // Test move out of pattern
+    struct Foo;
+    let _: (Foo, Foo) = match_vec!(vec![Some(Foo), Some(Foo)].into_iter();
+        [Some(f1), Some(f2)] => (f1, f2),
+    )
+    .unwrap();
+}