fixing "check" and adding dedicated comments and tests

run-tests.coffee

@@ -23,6 +23,7 @@ test_low_level = ->
 	test_assignments()
 	test_caching()
 	test_strings()
+	test_check()
 
 selftest  = ->
 	test_low_level()

sources/eval.coffee

@@ -265,16 +265,30 @@ Eval_cons = ->
 Eval_binding = ->
 	push(get_binding(cadr(p1)))
 
-# checks a predicate, i.e. check(A = B)
+### check =====================================================================
+
+Tags
+----
+scripting, JS, internal, treenode, general concept
+
+Parameters
+----------
+p
+
+General description
+-------------------
+Checks the predicate p, e.g. check(a = b)
+Note how "check" can turn what normally would be an assignment into a test,
+so in the case above "a" is not assigned anything.
+
+###
 
 # check definition
 Eval_check = ->
 	push(cadr(p1))
 	Eval_predicate()
 	p1 = pop()
-	if (iszero(p1))
-		stop("check(arg): arg is zero")
-	push(symbol(NIL)) # no result is printed
+	push(p1)
 
 Eval_det = ->
 	push(cadr(p1))

tests/check.coffee

@@ -0,0 +1,51 @@
+test_check = ->
+	if DEBUG then console.log "test_check ----------------------------"
+
+	run_test [
+
+		# note how check can turn an assignment
+		# into a test, so in this case a is not
+		# assigned anything
+
+		"check(a=b)",
+		"0",
+
+		"a",
+		"a",
+
+		"check(a=a)",
+		"1",
+
+		"check(a==a)",
+		"1",
+
+		"check(a===a)",
+		"check(a=== ? a)\nStop: syntax error",
+
+		"check(a+1=a)",
+		"0",
+
+		"check(a+1==a)",
+		"0",
+
+		"check(a+1===a)",
+		"check(a+1=== ? a)\nStop: syntax error",
+
+		"check(1)",
+		"1",
+
+		"check(0)",
+		"0",
+
+		"check(and(1,0))",
+		"0",
+
+		"check(and(1,1))",
+		"1",
+
+		# if not a predicate, just return
+		# what passed
+		"check(pi)",
+		"pi",
+
+	]