1 files changed, 54 insertions, 0 deletions
diff --git a/src/fold.zig b/src/fold.zig
new file mode 100644
index 0000000..6b910d3
--- /dev/null
+++ b/src/fold.zig
@@ -0,0 +1,54 @@
+const std = @import("std");
+const typeVerify = @import("util.zig").typeVerify;
+
+/// ```zig
+/// (fn (fn (fn (b, a) b, b, []a) void)
+/// ```
+/// Folds a list of items over a function with the accumulator as the first arg
+/// Type signature: `(b -> a -> b) -> b -> [a] -> b`
+/// `func` is of type `b -> a -> b`, where `items` is of type `[a]` and `accumulator` is of type `b`.
+/// Haskell equivalent: `foldl func accumulator items`
+pub fn foldl(
+    comptime func: anytype,
+    accumulator: (typeVerify(@TypeOf(func), .{ .@"fn" }).@"fn".params[0].type.?),
+    items: ([]typeVerify(@TypeOf(func), .{ .@"fn" }).@"fn".params[1].type.?),
+) typeVerify(@TypeOf(func), .{ .@"fn" }).@"fn".return_type.? {
+    var accum_internal = accumulator;
+    for(items) |i|
+        accum_internal = func(accum_internal, i);
+    return accum_internal;
+}
+
+/// ```zig
+/// (fn (fn (fn (a, b) b, b, []a) void)
+/// ```
+/// Folds a list of items over a function with the accumulator as the second arg
+/// Type signature: `(a -> b -> b) -> b -> [a] -> b`
+/// `func` is of type `a -> b -> b`, where `items` is of type `[a]` and `accumulator` is of type `b`.
+/// Haskell equivalent: `foldr func accumulator items`
+pub fn foldr(
+    comptime func: anytype,
+    accumulator: (typeVerify(@TypeOf(func), .{ .@"fn" }).@"fn".params[1].type.?),
+    items: ([]typeVerify(@TypeOf(func), .{ .@"fn" }).@"fn".params[0].type.?),
+) typeVerify(@TypeOf(func), .{ .@"fn" }).@"fn".return_type.? {
+    var accum_internal = accumulator;
+    for(items) |i|
+        accum_internal = func(i, accum_internal);
+    return accum_internal;
+}
+
+/// Variant of `foldl` where the first element is the base case
+pub fn foldl1(
+    comptime func: anytype,
+    items: ([]typeVerify(@TypeOf(func), .{.@"fn"}).@"fn".params[0].type.?)
+) typeVerify(@TypeOf(func), .{ .@"fn" }).@"fn".return_type.? {
+    return foldl(func, items[0], items);
+}
+
+/// Variant of `foldr` where the first element is the base case
+pub fn foldr1(
+    comptime func: anytype,
+    items: ([]typeVerify(@TypeOf(func), .{.@"fn"}).@"fn".params[0].type.?)
+) typeVerify(@TypeOf(func), .{ .@"fn" }).@"fn".return_type.? {
+    return foldr(func, items[0], items);
+}