proposed cat example implementation

example/cat/main.zig

@@ -1,7 +1,52 @@
 export executable "cat";
 
-pub fn main(argv: [][]u8) i32 => {
+import "std.zig";
 
+pub fn main(args: [][]u8) error => {
+    const exe = args[0];
+    var catted_anything = false;
+    for (arg in args[1...]) {
+        if (arg == "-") {
+            catted_anything = true;
+            cat_stream(stdin) !! (err) => return err;
+        } else if (arg[0] == '-') {
+            return usage(exe);
+        } else {
+            var is: InputStream;
+            is.open(arg, OpenReadOnly) !! (err) => {
+                stderr.print("Unable to open file: {}", ([]u8])(err));
+                return err;
+            }
+            defer is.close();
 
-    return 0;
+            catted_anything = true;
+            cat_stream(is) !! (err) => return err;
+        }
+    }
+    if (!catted_anything) {
+        cat_stream(stdin) !! (err) => return err;
+    }
+}
+
+fn usage(exe: []u8) error => {
+    stderr.print("Usage: {} [FILE]...\n");
+    return error.Invalid;
+}
+
+fn cat_stream(is: InputStream) error => {
+    var buf: [1024 * 4]u8;
+
+    while (true) {
+        const bytes_read = is.read(buf);
+        if (bytes_read < 0) {
+            stderr.print("Unable to read from stream: {}", ([]u8)(is.err));
+            return is.err;
+        }
+
+        const bytes_written = stdout.write(buf[0...bytes_read]);
+        if (bytes_written < bytes_read) {
+            stderr.print("Unable to write to stdout: {}", ([]u8)(stdout.err));
+            return stdout.err;
+        }
+    }
 }