semigrp: improve viewing of empty semigroups

Previously, unexpected errors could occur when viewing empty semigroups.
gap-system · Nov 26, 2018 · 23f47b6 · 23f47b6
1 parent 0a9548e
commit 23f47b6
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Showing 5 changed files with 29 additions and 9 deletions.
diff --git a/lib/magma.gi b/lib/magma.gi
@@ -1255,6 +1255,15 @@ InstallMethod( AsSubmagma,
     end );
 
 
+#############################################################################
+##
+#M  IsEmpty( <M> )  . . . . . . . . . . . . . . test whether a magma is empty
+##
+InstallMethod(IsEmpty, "for a magma with generators of magma",
+[IsMagma and HasGeneratorsOfMagma],
+M -> IsEmpty(GeneratorsOfMagma(M)));
+
+
 #############################################################################
 ##
 #E

diff --git a/lib/semigrp.gi b/lib/semigrp.gi
@@ -38,7 +38,9 @@ function(S)
 
   str := "\><";
 
-  if HasIsTrivial(S) and IsTrivial(S) then
+  if IsEmpty(S) then
+    Append(str, "\>empty\< ");
+  elif HasIsTrivial(S) and IsTrivial(S) then
     Append(str, "\>trivial\< ");
   else
     if HasIsFinite(S) and not IsFinite(S) then
@@ -49,7 +51,7 @@ function(S)
     fi;
   fi;
 
-  if not IsGroup(S) then
+  if not IsGroup(S) and not IsEmpty(S) then
     if HasIsTrivial(S) and IsTrivial(S) then
       # do nothing
     elif HasIsZeroSimpleSemigroup(S) and IsZeroSimpleSemigroup(S) then
@@ -72,7 +74,10 @@ function(S)
 
   Append(str, SemigroupViewStringPrefix(S));
 
-  if HasIsMonoid(S) and IsMonoid(S) then
+  if IsEmpty(S) then
+    Append(str, "\>semigroup\<>\<");
+    return str;
+  elif HasIsMonoid(S) and IsMonoid(S) then
     Append(str, "\>monoid\< ");
     if HasGeneratorsOfInverseMonoid(S) then
       nrgens := Length(GeneratorsOfInverseMonoid(S));
@@ -108,9 +113,7 @@ function(S)
   if nrgens > 1 or nrgens = 0 then
     Append(str, "s");
   fi;
-  Append(str, "\<");
-
-  Append(str, ">\<");
+  Append(str, "\<>\<");
 
   return str;
 end);

diff --git a/tst/testbugfix/2012-11-21-t00263.tst b/tst/testbugfix/2012-11-21-t00263.tst
@@ -10,7 +10,7 @@ true
 gap> Size(t);
 infinity
 gap> t := Subsemigroup(s, []);
-<semigroup of size 0, with 0 generators>
+<empty semigroup>
 gap> HasSize(t);
 true
 gap> Size(t);

diff --git a/tst/testbugfix/2018-05-09-submagma.tst b/tst/testbugfix/2018-05-09-submagma.tst
@@ -40,7 +40,7 @@ false
 
 #
 gap> sub:=Submagma(mgm,[]);
-<commutative semigroup with 0 generators>
+<empty semigroup>
 gap> Size(sub);
 0
 gap> IsTrivial(sub);

diff --git a/tst/testinstall/semigrp.tst b/tst/testinstall/semigrp.tst
@@ -395,7 +395,7 @@ gap> GeneratorsOfInverseMonoid(S);
 gap> S := FreeSemigroup(2);
 <free semigroup on the generators [ s1, s2 ]>
 gap> T := Subsemigroup(S, []);
-<semigroup of size 0, with 0 generators>
+<empty semigroup>
 gap> IsTrivial(T);
 false
 gap> IsEmpty(T);
@@ -501,6 +501,14 @@ Rank 2:  [H size = 1, 1 L-class, 1 R-class]
 Rank 2:  [H size = 1, 1 L-class, 1 R-class]
 Rank 1: *[H size = 1, 1 L-class, 1 R-class]
 
+# test printing an empty semigroup
+gap> S := FullTransformationMonoid(1);;
+gap> Subsemigroup(S, []);
+<empty transformation semigroup>
+gap> S := SymmetricInverseMonoid(1);;
+gap> Subsemigroup(S, []);
+<empty partial perm semigroup>
+
 #
 gap> STOP_TEST( "semigrp.tst", 1);