Fix empty terms in the binom function

Closes: #31

Author: runarberg

src/parser.js

@@ -165,8 +165,8 @@ function parser(options) {
     } else if (ascii.startsWith("binom")) {
       const [, , group, , after] = syntax.splitNextGroup(ascii);
       const [a, b] = colsplit(group);
-      const over = parsegroup(a.trim());
-      const under = parsegroup(b.trim());
+      const over = parsegroup(a.trim()) || mrow("");
+      const under = parsegroup(b.trim()) || mrow("");
 
       el = mfenced(mfrac(over + under, { linethickness: 0 }), {
         open: "(",

test/groupings.js

@@ -79,6 +79,32 @@ test("Complex groupings", t => {
   );
 });
 
+test("Binom function", t => {
+  t.is(
+    a2ml("binom(n, k)"),
+    '<math><mfenced open="(" close=")"><mfrac linethickness="0"><mi>n</mi><mi>k</mi></mfrac></mfenced></math>'
+  );
+});
+
+test("Binom function accepts expressions", t => {
+  t.is(
+    a2ml("binom(a, b + c)"),
+    '<math><mfenced open="(" close=")"><mfrac linethickness="0"><mi>a</mi><mrow><mi>b</mi><mo>+</mo><mi>c</mi></mrow></mfrac></mfenced></math>'
+  );
+});
+
+test("Missing argument in the binom function", t => {
+  t.is(
+    a2ml("binom(a,)"),
+    '<math><mfenced open="(" close=")"><mfrac linethickness="0"><mi>a</mi><mrow></mrow></mfrac></mfenced></math>'
+  );
+
+  t.is(
+    a2ml("binom(,b)"),
+    '<math><mfenced open="(" close=")"><mfrac linethickness="0"><mrow></mrow><mi>b</mi></mfrac></mfenced></math>'
+  );
+});
+
 test("Simplify polynomials", t => {
   t.is(
     a2ml("(x+y)(x-y) = x^2-y^2"),
@@ -113,3 +139,10 @@ test("Average over time", t => {
     '<math><mfenced open="⟨" close="⟩"><mrow><mi>V</mi><msup><mfenced open="(" close=")"><mi>t</mi></mfenced><mn>2</mn></msup></mrow></mfenced><mo>=</mo><munder><mi>lim</mi><mrow><mi>T</mi><mo>→</mo><mi mathvariant="normal">∞</mi></mrow></munder><mfrac><mn>1</mn><mi>T</mi></mfrac><msubsup><mo>∫</mo><mrow><mo>-</mo><mfrac bevelled="true"><mi>T</mi><mn>2</mn></mfrac></mrow><mfrac bevelled="true"><mi>T</mi><mn>2</mn></mfrac></msubsup><mi>V</mi><msup><mfenced open="(" close=")"><mi>t</mi></mfenced><mn>2</mn></msup><mi>d</mi><mi>t</mi></math>'
   );
 });
+
+test("The binomial coefficient", t => {
+  t.is(
+    a2ml("binom(n, k) = n! / (n-k)!k!"),
+    '<math><mfenced open="(" close=")"><mfrac linethickness="0"><mi>n</mi><mi>k</mi></mfrac></mfenced><mo>=</mo><mfrac><mrow><mi>n</mi><mo>!</mo></mrow><mrow><mfenced open="(" close=")"><mrow><mi>n</mi><mo>-</mo><mi>k</mi></mrow></mfenced><mo>!</mo><mi>k</mi><mo>!</mo></mrow></mfrac></math>'
+  );
+});

test/roots.js

@@ -14,20 +14,6 @@ test("Omits brackets in roots", t => {
   t.is(a2ml("root(3)(2)"), "<math><mroot><mn>2</mn><mn>3</mn></mroot></math>");
 });
 
-test("Display the binomial coefficient", t => {
-  t.is(
-    a2ml("binom(n, k)"),
-    '<math><mfenced open="(" close=")"><mfrac linethickness="0"><mi>n</mi><mi>k</mi></mfrac></mfenced></math>'
-  );
-});
-
-test("Displays the whole terms of the binomial coefficient", t => {
-  t.is(
-    a2ml("binom(a, b + c)"),
-    '<math><mfenced open="(" close=")"><mfrac linethickness="0"><mi>a</mi><mrow><mi>b</mi><mo>+</mo><mi>c</mi></mrow></mfrac></mfenced></math>'
-  );
-});
-
 test("Allows empty roots", t => {
   t.is(a2ml("sqrt"), "<math><msqrt><mrow></mrow></msqrt></math>");
   t.is(
@@ -73,10 +59,3 @@ test("Continued square root", t => {
     "<math><msqrt><mrow><mn>1</mn><mo>+</mo><msqrt><mrow><mn>1</mn><mo>+</mo><msqrt><mrow><mn>1</mn><mo>+</mo><msqrt><mrow><mn>1</mn><mo>+</mo><msqrt><mrow><mn>1</mn><mo>+</mo><msqrt><mrow><mn>1</mn><mo>+</mo><msqrt><mrow><mn>1</mn><mo>+</mo><mo>⋯</mo></mrow></msqrt></mrow></msqrt></mrow></msqrt></mrow></msqrt></mrow></msqrt></mrow></msqrt></mrow></msqrt></math>"
   );
 });
-
-test("The binomial coefficient", t => {
-  t.is(
-    a2ml("binom(n, k) = n! / (n-k)!k!"),
-    '<math><mfenced open="(" close=")"><mfrac linethickness="0"><mi>n</mi><mi>k</mi></mfrac></mfenced><mo>=</mo><mfrac><mrow><mi>n</mi><mo>!</mo></mrow><mrow><mfenced open="(" close=")"><mrow><mi>n</mi><mo>-</mo><mi>k</mi></mrow></mfenced><mo>!</mo><mi>k</mi><mo>!</mo></mrow></mfrac></math>'
-  );
-});