libgap: add GAP_IsChar, fix GAP_IsMacFloat, expand some comments

- `GAP_IsMacFloat` now returns an `int` instead of `Int`, like all the
  other `GAP_Is*` functions (technically this is an ABI breaking change)
- `GAP_IsMacFloat` now also accepts a null pointer
- add some comments for functions that are safe to call without
  catching GAP errors, because they never raise an error (so this is
  a binding API/ABI promise)
- add `GAP_IsChar` (note that technically one could already do this
  by checking `GAP_ValueOfChar(obj) != -1`)

src/libgap-api.c

@@ -313,9 +313,9 @@ Obj GAP_CallFunc3Args(Obj func, Obj a1, Obj a2, Obj a3)
 //// floats
 ////
 
-Int GAP_IsMacFloat(Obj obj)
+int GAP_IsMacFloat(Obj obj)
 {
- return IS_MACFLOAT(obj);
+ return obj && IS_MACFLOAT(obj);
 }
 
 double GAP_ValueMacFloat(Obj obj)
@@ -564,6 +564,11 @@ char * GAP_CSTR_STRING(Obj string)
  return CSTR_STRING(string);
 }
 
+int GAP_IsChar(Obj obj)
+{
+ return obj && TNUM_OBJ(obj) == T_CHAR;
+}
+
 Int GAP_ValueOfChar(Obj obj)
 {
  if (TNUM_OBJ(obj) != T_CHAR) {

src/libgap-api.h

@@ -319,7 +319,9 @@ Obj GAP_CallFunc3Args(Obj func, Obj a1, Obj a2, Obj a3);
 ////
 
 // Returns 1 if <obj> is a GAP machine float, 0 if not.
-Int GAP_IsMacFloat(Obj obj);
+//
+// Never raises an error. Safe to be called with a NULL pointer.
+int GAP_IsMacFloat(Obj obj);
 
 // Returns the value of the GAP machine float object <obj>.
 // If <obj> is not a machine float object, an error is raised.
@@ -334,12 +336,18 @@ Obj GAP_NewMacFloat(double x);
 ////
 
 // Returns 1 if <obj> is a GAP integer, 0 if not.
+//
+// Never raises an error. Safe to be called with a NULL pointer.
 int GAP_IsInt(Obj obj);
 
 // Returns 1 if <obj> is a GAP small (aka immediate) integer, 0 if not.
+//
+// Never raises an error. Safe to be called with a NULL pointer.
 int GAP_IsSmallInt(Obj obj);
 
 // Returns 1 if <obj> is a GAP large integer, 0 if not.
+//
+// Never raises an error. Safe to be called with a NULL pointer.
 int GAP_IsLargeInt(Obj obj);
 
 
@@ -355,6 +363,8 @@ int GAP_IsLargeInt(Obj obj);
 Obj GAP_MakeObjInt(const UInt * limbs, Int size);
 
 // Return a GAP integer object with value equal to <val>.
+//
+// Never raises an error.
 Obj GAP_NewObjIntFromInt(Int val);
 
 // Return an integer equal to the given GAP integer object. If <obj> is not
@@ -387,6 +397,8 @@ const UInt * GAP_AddrInt(Obj obj);
 ////
 
 // Returns 1 if <obj> is a GAP list, 0 if not.
+//
+// Never raises an error. Safe to be called with a NULL pointer.
 int GAP_IsList(Obj obj);
 
 // Returns the length of the given GAP list.
@@ -433,12 +445,18 @@ Obj GAP_NewRange(Int len, Int low, Int inc);
 // `GAP_IsMatrixObj` checks for special cases that are not plain lists.
 
 // Returns 1 if <obj> is a GAP matrix or matrix obj, 0 if not.
+//
+// Never raises an error. Safe to be called with a NULL pointer.
 int GAP_IsMatrixOrMatrixObj(Obj obj);
 
 // Returns 1 if <obj> is a GAP matrix, 0 if not.
+//
+// Never raises an error. Safe to be called with a NULL pointer.
 int GAP_IsMatrix(Obj obj);
 
 // Returns 1 if <obj> is a GAP matrix obj, 0 if not.
+//
+// Never raises an error. Safe to be called with a NULL pointer.
 int GAP_IsMatrixObj(Obj obj);
 
 // Returns the number of rows of the given GAP matrix or matrix obj.
@@ -466,6 +484,8 @@ Obj GAP_ElmMat(Obj mat, UInt row, UInt col);
 ////
 
 // Returns 1 if <obj> is a GAP record, 0 if not.
+//
+// Never raises an error. Safe to be called with a NULL pointer.
 int GAP_IsRecord(Obj obj);
 
 // Assign <val> to component given by <name> in the GAP record <rec>.
@@ -487,6 +507,8 @@ Obj GAP_NewPrecord(Int capacity);
 ////
 
 // Returns 1 if <obj> is a GAP string, 0 if not.
+//
+// Never raises an error. Safe to be called with a NULL pointer.
 int GAP_IsString(Obj obj);
 
 // Returns the length of the given GAP string.
@@ -524,11 +546,25 @@ Obj GAP_MakeStringWithLen(const char * string, UInt len);
 // terminated C string.
 Obj GAP_MakeImmString(const char * string);
 
+
+////
+//// chars
+////
+
+// Returns 1 if <obj> is a GAP char, 0 if not.
+//
+// Never raises an error. Safe to be called with a NULL pointer.
+int GAP_IsChar(Obj obj);
+
 // Returns the value of the GAP character object <obj>.
 // If <obj> is not a GAP character object, it returns -1.
+//
+// Never raises an error.
 Int GAP_ValueOfChar(Obj obj);
 
 // Returns the GAP character object with value <obj>.
+//
+// Never raises an error.
 Obj GAP_CharWithValue(UChar obj);
 
 #ifdef __cplusplus