diff --git a/tests/valid/Mmake b/tests/valid/Mmake
index 47f158ffe..925f8fae0 100644
--- a/tests/valid/Mmake
+++ b/tests/valid/Mmake
@@ -8,25 +8,42 @@ include ../Mmake
 
 #-----------------------------------------------------------------------------#
 
-SOURCES=			\
-	assoc_list.nl		\
-	det_inference.nl	\
-	det_switch.nl		\
-	easy_nondet_test.nl	\
-	easy_nondet_test_2.nl	\
-	mode_merge_insts.nl	\
-	name_mangling.nl	\
-	same_length_2.nl	\
-	same_length_3.nl	\
-	indexing.nl		\
-	tiny.nl
+SOURCES= \
+	complicated_unify.m \
+	dcg_test.m \
+	det_condition.m \
+	det_inference.m \
+	det_switch.m \
+	easy_nondet_test.m \
+	easy_nondet_test_2.m \
+	error.m \
+	followcode_det_problem.m \
+	higher_order.m \
+	implied_mode.m \
+	indexing.m \
+	loop.m \
+	name_mangling.m \
+	semidet_disj.m \
+	stack_alloc.m \
+	switches.m \
+	unreachable_code.m
 
-OBJS=	$(SOURCES:%.nl=%.o)
-NOS =	$(SOURCES:%.nl=%.no)
-PROGS =	$(SOURCES:%.nl=%)
+# The mode system can't handle the following test cases yet:
+#	assoc_list.m
+#	determinism.m
+#	mode_merge_insts.m
+# There is a bug which prevents us from passing this test case:
+#	same_length_2.m
 
-depend: tiny.depend
+OBJS=	$(SOURCES:%.m=%.o)
+NOS =	$(SOURCES:%.m=%.no)
+PROGS =	$(SOURCES:%.m=%)
 
-all:	$(OBJS) tiny
+all: objs
+check: objs
+objs: $(OBJS)
+depend:
+clean:
+	rm -f *.c *.o *.err2 *.d
 
 #-----------------------------------------------------------------------------#
diff --git a/tests/valid/assoc_list.nl b/tests/valid/assoc_list.m
similarity index 80%
rename from tests/valid/assoc_list.nl
rename to tests/valid/assoc_list.m
index 8cbde3f61..d8731a554 100644
--- a/tests/valid/assoc_list.nl
+++ b/tests/valid/assoc_list.m
@@ -6,7 +6,7 @@
 
 :- pred assoc_list_member(pair(K,V), list(pair(K,V))).
 :- mode assoc_list_member(bound(free - ground) -> ground, in) is semidet.
-:- mode assoc_list_member(bound(free - free) -> ground, in) is det.
+:- mode assoc_list_member(bound(free - free) -> ground, in) is nondet.
 
 assoc_list_member(X, [X|_]).
 assoc_list_member(X, [_|Xs]) :-
diff --git a/tests/valid/complicated_unify.nl b/tests/valid/complicated_unify.m
similarity index 100%
rename from tests/valid/complicated_unify.nl
rename to tests/valid/complicated_unify.m
diff --git a/tests/valid/dcg_test.nl b/tests/valid/dcg_test.m
similarity index 82%
rename from tests/valid/dcg_test.nl
rename to tests/valid/dcg_test.m
index fc1135221..7d959f01a 100644
--- a/tests/valid/dcg_test.nl
+++ b/tests/valid/dcg_test.m
@@ -6,6 +6,12 @@
 :- pred q is semidet.
 :- pred r is semidet.
 :- pred s is semidet.
+:- external(q/2).
+:- external(r/2).
+:- external(s/2).
+:- external(q/0).
+:- external(r/0).
+:- external(s/0).
 
 :- pred p(int::in, int::out) is nondet.
 
diff --git a/tests/valid/det_condition.nl b/tests/valid/det_condition.m
similarity index 100%
rename from tests/valid/det_condition.nl
rename to tests/valid/det_condition.m
diff --git a/tests/valid/det_inference.nl b/tests/valid/det_inference.m
similarity index 88%
rename from tests/valid/det_inference.nl
rename to tests/valid/det_inference.m
index 6ba8250f3..ff8ea2abc 100644
--- a/tests/valid/det_inference.nl
+++ b/tests/valid/det_inference.m
@@ -1,6 +1,6 @@
 :- module det_inference.
 
-:- pred p1.
+:- pred p1 is det.
 :- pred p2.
 :- pred p3.
 :- pred p4.
diff --git a/tests/valid/det_switch.nl b/tests/valid/det_switch.m
similarity index 88%
rename from tests/valid/det_switch.nl
rename to tests/valid/det_switch.m
index 5e5661e3d..8d113334c 100644
--- a/tests/valid/det_switch.nl
+++ b/tests/valid/det_switch.m
@@ -5,7 +5,7 @@
 :- pred p(enum, int) is det.
 :- mode p(in, out) is det.
 :- mode p(out, in) is semidet.
-:- mode p(out, out) is nondet.
+:- mode p(out, out) is multi.
 
 p(a, 1).
 p(b, 2).
diff --git a/tests/valid/determinism.nl b/tests/valid/determinism.m
similarity index 99%
rename from tests/valid/determinism.nl
rename to tests/valid/determinism.m
index c45631d07..29c78098b 100644
--- a/tests/valid/determinism.nl
+++ b/tests/valid/determinism.m
@@ -37,7 +37,5 @@ p3 :-
 	r(Y),
 	X = Y.
 
-/*
 :- external(q/1).
 :- external(r/1).
-*/
diff --git a/tests/valid/easy_nondet_test.nl b/tests/valid/easy_nondet_test.m
similarity index 90%
rename from tests/valid/easy_nondet_test.nl
rename to tests/valid/easy_nondet_test.m
index 7403a5d06..9bcd1201e 100644
--- a/tests/valid/easy_nondet_test.nl
+++ b/tests/valid/easy_nondet_test.m
@@ -5,7 +5,7 @@
 q(X) :-
 	p(X).
 
-:- pred p(int::out) is nondet.
+:- pred p(int::out) is multi.
 
 p(X) :- 
 	X = 1 ; X = 2.
diff --git a/tests/valid/easy_nondet_test_2.nl b/tests/valid/easy_nondet_test_2.m
similarity index 82%
rename from tests/valid/easy_nondet_test_2.nl
rename to tests/valid/easy_nondet_test_2.m
index de00e97ac..5ab4c83ba 100644
--- a/tests/valid/easy_nondet_test_2.nl
+++ b/tests/valid/easy_nondet_test_2.m
@@ -4,13 +4,13 @@
 
 :- pred q(int::out) is nondet.
 
-% q(1).
-% q(2).
+q(1).
+q(2).
 
 :- pred r(int::out) is nondet.
 
-% r(3).
-% r(4).
+r(3).
+r(4).
 
 p(X) :-
 	q(X) ; r(X).
diff --git a/tests/valid/error.nl b/tests/valid/error.m
similarity index 90%
rename from tests/valid/error.nl
rename to tests/valid/error.m
index e1ac7a067..d8c2caf82 100644
--- a/tests/valid/error.nl
+++ b/tests/valid/error.m
@@ -2,6 +2,6 @@
 :- import_module require.
 
 :- pred t(int::out) is det.
-t(X) :-
+t(_X) :-
 	error("").
 
diff --git a/tests/valid/followcode_det_problem.nl b/tests/valid/followcode_det_problem.m
similarity index 100%
rename from tests/valid/followcode_det_problem.nl
rename to tests/valid/followcode_det_problem.m
index a572e38ac..eb2f5e2a1 100644
--- a/tests/valid/followcode_det_problem.nl
+++ b/tests/valid/followcode_det_problem.m
@@ -3,12 +3,12 @@
 :- pred p(int::out) is semidet.
 
 p(X) :-
+	Z = 4,
 	(
 		Z = 3
 	;
 		Z = 4
 	),
-	Z = 4,
 	q(X).
 
 :- pred q(int::out) is det.
diff --git a/tests/valid/higher_order.nl b/tests/valid/higher_order.m
similarity index 77%
rename from tests/valid/higher_order.nl
rename to tests/valid/higher_order.m
index 874596845..975ec8f94 100644
--- a/tests/valid/higher_order.nl
+++ b/tests/valid/higher_order.m
@@ -1,11 +1,8 @@
 :- module higher_order.
 :- import_module list, int.
 
-:- pred call(pred(T1, T2), T1, T2).
-:- mode call(in, in, out) is det.
-
 :- pred map_list(pred(T1, T2), list(T1), list(T2)).
-:- mode map_list(in, in, out) is det.
+:- mode map_list(pred(in, out) is det, in, out) is det.
 
 map_list(_P, [], []).
 map_list(P, [X|Xs], [Y|Ys]) :-
@@ -28,6 +25,6 @@ t.
 f :- fail.
 
 :- pred test(pred).
-:- mode test(out) is nondet.
+:- mode test(out((pred) is semidet)) is nondet.
 test(t).
 test(f).
diff --git a/tests/valid/implied_mode.nl b/tests/valid/implied_mode.m
similarity index 100%
rename from tests/valid/implied_mode.nl
rename to tests/valid/implied_mode.m
diff --git a/tests/valid/indexing.nl b/tests/valid/indexing.m
similarity index 100%
rename from tests/valid/indexing.nl
rename to tests/valid/indexing.m
diff --git a/tests/valid/loop.m b/tests/valid/loop.m
new file mode 100644
index 000000000..c5af986d1
--- /dev/null
+++ b/tests/valid/loop.m
@@ -0,0 +1,9 @@
+:- module loop.
+:- interface.
+:- import_module io.
+
+:- pred main(io__state::di, io__state::uo) is det.
+
+:- implementation.
+
+main --> main.
diff --git a/tests/valid/mode_merge_insts.nl b/tests/valid/mode_merge_insts.m
similarity index 62%
rename from tests/valid/mode_merge_insts.nl
rename to tests/valid/mode_merge_insts.m
index 6a4b48dd0..a2191ce6c 100644
--- a/tests/valid/mode_merge_insts.nl
+++ b/tests/valid/mode_merge_insts.m
@@ -2,15 +2,17 @@
 
 :- type triple ---> triple(int, int, int).
 
-:- type list(T) ---> [] ; [T | list(T)].
-
 :- inst triple1	= bound(triple(ground, free, free)).
 :- inst triple2	= bound(triple(free, ground, free)).
 :- inst triple12 = bound(triple(ground, ground, free)).
 
-:- pred p1(list(triple) :: free -> triple1) is det.
-:- pred p2(list(triple) :: free -> triple2) is det.
-:- pred q(list(triple) :: triple12 -> triple12) is det.
+:- pred p1(triple :: free -> triple1) is det.
+:- pred p2(triple :: free -> triple2) is det.
+:- pred q(triple :: triple12 -> triple12) is det.
+
+:- external(p1/1).
+:- external(p2/1).
+:- external(q/1).
 
 :- pred p is det.
 
diff --git a/tests/valid/name_mangling.nl b/tests/valid/name_mangling.m
similarity index 100%
rename from tests/valid/name_mangling.nl
rename to tests/valid/name_mangling.m
diff --git a/tests/valid/same_length_2.m b/tests/valid/same_length_2.m
new file mode 100644
index 000000000..f85c50fbb
--- /dev/null
+++ b/tests/valid/same_length_2.m
@@ -0,0 +1,28 @@
+:- module same_length_2.
+:- import_module list.
+
+:- mode my_input_list_skel :: list_skel -> list_skel.
+:- mode my_output_list_skel :: free -> list_skel.
+:- mode my_list_skel_output :: list_skel -> ground.
+
+:- pred q(list(T)::my_input_list_skel).
+:- pred r(list(T)::my_output_list_skel).
+
+:- external(q/1).
+:- external(r/1).
+
+:- pred p.
+
+p :-
+	r(X),
+	q(X).
+
+:- pred p2(list(T)::my_output_list_skel).
+
+p2(X) :-
+	r(X),
+	q(X)
+	;
+	r(X),
+	q(X).
+
diff --git a/tests/valid/same_length_2.nl b/tests/valid/same_length_2.nl
deleted file mode 100644
index b49f6297e..000000000
--- a/tests/valid/same_length_2.nl
+++ /dev/null
@@ -1,28 +0,0 @@
-:- module same_length_2.
-
-:- type list(T) ---> [] ; [T | list(T)].
-
-:- inst list_skel = bound([] ; [free | list_skel]).
-
-:- mode input_list_skel :: list_skel -> list_skel.
-:- mode output_list_skel :: free -> list_skel.
-:- mode list_skel_output :: list_skel -> ground.
-
-:- pred q(list(T)::input_list_skel).
-:- pred r(list(T)::output_list_skel).
-
-:- pred p.
-
-p :-
-	r(X),
-	q(X).
-
-:- pred p2(list(T)::output_list_skel).
-
-p2(X) :-
-	r(X),
-	q(X)
-	;
-	r(X),
-	q(X).
-
diff --git a/tests/valid/same_length_3.nl b/tests/valid/same_length_3.nl
deleted file mode 100644
index a2d2811ba..000000000
--- a/tests/valid/same_length_3.nl
+++ /dev/null
@@ -1,28 +0,0 @@
-:- module same_length_3.
-
-:- type list(T) ---> [] ; [T | list(T)].
-
-:- inst list_skel = bound([] ; [free | list_skel]).
-
-:- mode input_list_skel :: list_skel -> list_skel.
-:- mode output_list_skel :: free -> list_skel.
-:- mode list_skel_output :: list_skel -> ground.
-
-:- pred q(list(T)::input_list_skel) is det.
-:- pred r(list(T)::output_list_skel) is det.
-
-:- pred p is det.
-
-p :-
-	r(X),
-	q(X).
-
-:- pred p2(list(T)::output_list_skel) is nondet.
-
-p2(X) :-
-	r(X),
-	q(X)
-	;
-	r(X),
-	q(X).
-
diff --git a/tests/valid/semidet_disj.m b/tests/valid/semidet_disj.m
new file mode 100644
index 000000000..bc5eb0335
--- /dev/null
+++ b/tests/valid/semidet_disj.m
@@ -0,0 +1,11 @@
+:- module semidet_disj.
+
+:- pred p(int::in) is semidet.
+:- pred q(int::in) is semidet.
+:- external(q/1).
+:- pred r(int::in) is semidet.
+:- external(r/1).
+
+p(X) :-
+	q(X) ; r(X).
+
diff --git a/tests/valid/stack_alloc.nl b/tests/valid/stack_alloc.m
similarity index 84%
rename from tests/valid/stack_alloc.nl
rename to tests/valid/stack_alloc.m
index b7277f531..f38b221af 100644
--- a/tests/valid/stack_alloc.nl
+++ b/tests/valid/stack_alloc.m
@@ -3,6 +3,9 @@
 :- pred in(int::in) is semidet.
 :- pred out(int::out) is det.
 
+:- external(in/1).
+:- external(out/1).
+
 :- pred p is semidet.
 
 p :-
diff --git a/tests/valid/switches.nl b/tests/valid/switches.m
similarity index 100%
rename from tests/valid/switches.nl
rename to tests/valid/switches.m
diff --git a/tests/valid/tiny.nl b/tests/valid/tiny.nl
deleted file mode 100644
index a28bbf1e2..000000000
--- a/tests/valid/tiny.nl
+++ /dev/null
@@ -1,8 +0,0 @@
-:- module tiny.
-:- interface.
-:- import_module io.
-
-:- pred main(io::di, io::uo) is det.
-
-main --> [].
-
diff --git a/tests/valid/unreachable_code.m b/tests/valid/unreachable_code.m
new file mode 100644
index 000000000..fcffa28a8
--- /dev/null
+++ b/tests/valid/unreachable_code.m
@@ -0,0 +1,15 @@
+:- module unreachable_code.
+:- interface.
+
+:- type foo ---> foo.
+
+:- pred p(foo::in, foo::in) is semidet.
+
+:- implementation.
+:- import_module require.
+
+p(X, Y) :-
+	error("err"),
+	X = Y.
+
+%-----------------------------------------------------------------------------%