Conversion rules are not cooperating.

rules/rules.go

@@ -20,10 +20,12 @@ var (
 		unit_kind(c, temperature).
 		unit_kind(m2, area).
 		unit_kind(cm2, area).
+		unit_kind(mm, distance).
 		unit_kind(cm, distance).
 		unit_kind(m, distance).
 		unit_kind(km, distance).
 		unit_kind(feet, distance).
+		unit_kind(mg, mass).
 		unit_kind(g, mass).
 		unit_kind(kg, mass).
 		unit_kind(lb, mass).
@@ -41,13 +43,36 @@ rule(GivenQuant, c, TargetQuant, k) :-
 			TargetQuant is GivenQuant + 273.15.
 		rule(GivenQuant, k, TargetQuant, c) :-
 			TargetQuant is GivenQuant - 273.15.
+
 		rule(GivenQuant, GivenUnit, TargetQuant, TargetUnit) :-
 			rule(GivenQuant, GivenUnit, X, Y),
 			rule(X, Y, TargetQuant, TargetUnit).
+
+		ratio(m, km, 0.001).
+		ratio(m, cm, 100).
+		ratio(m, mm, 1000).
+		ratio(feet, m, 0.3048).
+		ratio(feet, inches, 12).
+
+		ratio(g, mg, 1000).
+		ratio(g, kg, 0.001).
+
+		ratio(X, Y, Ratio) :- ratio(Y, X, Z), Ratio is 1/Z, !.
+		ratio(X, Y, Ratio) :-
+			ratio(Y, Step, R1),
+		 	ratio(Step, Z, R2),
+		 	Step =\= Y,
+		 	Step =\= Z,
+		 	Ratio is R1 * R2, !.
+
 		convert(GivenQuant, GivenUnit, TargetQuant, TargetUnit) :-
 			unit_kind(GivenUnit, temperature),
 			unit_kind(TargetUnit, temperature),
 			rule(GivenQuant, GivenUnit, TargetQuant, TargetUnit).
+		convert(GivenQuant, GivenUnit, TargetQuant, TargetUnit) :-
+			write("foo"), nl,
+			ratio(GivenUnit, TargetUnit, Ratio),
+			TargetQuant is GivenQuant * Ratio, !.
 `
 )
 
@@ -60,6 +85,9 @@ func New() (*Engine, error) {
 	if err := ngn.p.Exec(`:- set_prolog_flag(double_quotes, atom).`); err != nil {
 		return nil, err
 	}
+	if err := ngn.p.Exec(`:- set_prolog_flag(debug, on).`); err != nil {
+		return nil, err
+	}
 
 	if err := ngn.p.Exec(prologKinds); err != nil {
 		return nil, err
@@ -98,22 +126,31 @@ func (ngn *Engine) Kind(unit string) (string, error) {
 
 // Convert converts units.
 func (ngn *Engine) Convert(qty float64, givenUnit, targetUnit string) (float64, error) {
-	sols, err := ngn.p.Query("convert(?, ?, Qty, ?).", qty, givenUnit, targetUnit)
+	sol := ngn.p.QuerySolution("convert(?, ?, Qty, ?).", qty, givenUnit, targetUnit)
+	if sol == nil {
+		fmt.Println("sol == nil")
+		return 0, errors.New("Query failed")
+	}
+	err := sol.Err()
 	if err != nil {
-		fmt.Println("Huh?")
+		fmt.Println("sol.Err()")
 		return 0, err
 	}
-	defer sols.Close()
 
-	for sols.Next() {
 	var quantity struct {
 		Qty float64
 	}
-		if err := sols.Scan(&quantity); err != nil {
-			fmt.Println("Scan?")
-			return 0, err
-		}
+	err = sol.Scan(&quantity)
+	if err == nil {
+		fmt.Println("got qty")
 		return quantity.Qty, err
 	}
+	fmt.Println("qty scan failed")
+	var dbg []byte
+	if err := sol.Scan(&dbg); err != nil {
+		fmt.Println("on dbg scan")
+		return 0, err
+	}
+	fmt.Printf("dbg: '%s'\n", dbg)
 	return 0, err
 }

rules/rules_test.go

@@ -4,6 +4,7 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestKinds(t *testing.T) {
@@ -20,11 +21,11 @@ func TestKinds(t *testing.T) {
 	}
 
 	ngn, err := New()
-	assert.NoError(t, err)
+	require.NoError(t, err)
 	for _, tc := range cases {
 		t.Run(tc.name, func(t *testing.T) {
 			result, err := ngn.Kind(tc.query)
-			assert.NoError(t, err)
+			require.NoError(t, err)
 
 			assert.Equal(t, tc.result, result)
 		})
@@ -36,31 +37,32 @@ func TestConverts(t *testing.T) {
 		name   string
 		qty    float64
 		given  string
-		target string
 		result float64
+		target string
 	}{
 		{
 			"c to f",
-			22.0,
-			"c",
-			"f",
-			71.6,
+			22.0, "c",
+			71.6, "f",
 		},
 		{
 			"k to f",
-			22.0,
-			"k",
-			"f",
-			-420.07,
+			22.0, "k",
+			-420.07, "f",
+		},
+		{
+			"inches to cm",
+			2.0, "inches",
+			5.08, "cm",
 		},
 	}
 
 	ngn, err := New()
-	assert.NoError(t, err)
+	require.NoError(t, err)
 	for _, tc := range cases {
 		t.Run(tc.name, func(t *testing.T) {
 			result, err := ngn.Convert(tc.qty, tc.given, tc.target)
-			assert.NoError(t, err)
+			require.NoError(t, err)
 
 			assert.InDelta(t, tc.result, result, 0.1)
 		})