Fix prelude head-or` (#182)

- fix: prelude function `head-or` now works

Author: gmorpheme

harness/test/051_head_or.eu

@@ -0,0 +1,10 @@
+"Tests for prelude `head-or`"
+
+head-or-tests: {
+  α: [] head-or(:none) //= :none
+  β: [:x, :y, :z] head-or(null) //= :x
+  ɣ: { colours: [:r, :o, :y, :g, :b, :i, :v] }.{ val: head-or(:x, colours) = head(colours) }.val
+  δ: [null] head //= [] head-or(null)
+}
+
+RESULT: head-or-tests values all-true? then(:PASS, :FAIL)

lib/prelude.eu

@@ -68,7 +68,7 @@ head: __HEAD
 nil?: = []
 
 ` "`head-or(xs, d)` - return the head item of list `xs` or default `d` if empty."
-head-or(d, xs): xs nil? then(xs head, d)
+head-or(d, xs): xs nil? then(d, xs head)
 
 ` "`tail(xs)` - return list `xs` without the head item. [] causes error."
 tail: __TAIL

tests/harness_test.rs

@@ -245,3 +245,8 @@ pub fn test_harness_049() {
 pub fn test_harness_050() {
     run_test(&opts("050_edn.edn"));
 }
+
+#[test]
+pub fn test_harness_051() {
+    run_test(&opts("051_head_or.eu"));
+}