To gap string

src/present.cpp

@@ -1581,16 +1581,18 @@ modified version.
       * :any:`is_strongly_compressible`
 
 )pbdoc");
+
       m.def(
           "presentation_to_gap_string",
           [](Presentation_ const& p, std::string const& var_name) {
             return presentation::to_gap_string(p, var_name);
           },
           py::arg("p"),
-          py::arg("var_name"),
+          py::arg("var_name") = "S",
           R"pbdoc(
 :sig=(p: Presentation, var_name: str) -> str:
 :only-document-once:
+
 Return the code that would create *p* in GAP.
 
 This function returns the string of GAP code that could be used to create an
@@ -1600,12 +1602,26 @@ are created by taking quotients of free semigroups or monoids.
 :param p: the presentation.
 :type p: Presentation
 
-:param var_name: the name of the variable to be used in GAP.
+:param var_name: the name of the variable to be used in GAP (defaults to ``"S"``).
 :type var_name: str
 
 :returns: The GAP string.
 :rtype: str
+
+.. doctest::
+
+    >>> from libsemigroups_pybind11 import Presentation, presentation
+    >>> p = Presentation("ab")
+    >>> presentation.add_rule(p, "ab", "ba")
+    >>> print(presentation.to_gap_string(p), end="")
+    F := FreeSemigroup("a", "b");
+    AssignGeneratorVariables(F);;
+    R := [
+           [a * b, b * a]
+         ];
+    S := F / R;
 )pbdoc");
+
       m.def(
           "presentation_to_ace_string",
           [](Presentation_ const& p) { return presentation::to_ace_string(p); },

tests/test_present.py

@@ -819,6 +819,38 @@ def check_to_ace_string(W):
     )
 
 
+def check_to_gap_string(W):
+    p = Presentation(W([0, 1]))
+    presentation.add_rule(p, W([0, 1]), W([1, 0]))
+
+    a, b = ("s0", "s1") if W is to_word else ("a", "b")
+    assert (
+        presentation.to_gap_string(p)
+        == f"""F := FreeSemigroup("{a}", "{b}");
+AssignGeneratorVariables(F);;
+R := [
+       [{a} * {b}, {b} * {a}]
+     ];
+S := F / R;
+"""
+    )
+
+    a, b = ("m0", "m1") if W is to_word else ("a", "b")
+    p.contains_empty_word(True)
+    presentation.add_rule(p, W([0, 0]), W([]))
+    assert (
+        presentation.to_gap_string(p, "M")
+        == f"""F := FreeMonoid("{a}", "{b}");
+AssignGeneratorVariables(F);;
+R := [
+       [{a} * {b}, {b} * {a}],
+       [{a} * {a}, One(F)]
+     ];
+M := F / R;
+"""
+    )
+
+
 ###############################################################################
 # Test functions begin
 ###############################################################################
@@ -1780,3 +1812,8 @@ def test_add_idempotent_rules():
 def test_to_ace_string():
     check_to_ace_string(to_word)
     check_to_ace_string(to_string)
+
+
+def test_to_gap_string():
+    check_to_gap_string(to_word)
+    check_to_gap_string(to_string)