sequences: require non-negative-integer for iotas.

core/sequences/sequences-tests.factor

@@ -313,6 +313,7 @@ M: bogus-hashcode hashcode* 2drop 0 >bignum ;
 [ V{ 0 3 } ] [ "A" { "A" "B" "C" "A" "D" } indices ] unit-test
 
 [ "asdf" iota ] must-fail
+[ -1 iota ] must-fail
 [ T{ iota { n 10 } } ] [ 10 iota ] unit-test
 [ 0 ] [ 10 iota first ] unit-test
 
@@ -374,4 +375,3 @@ USE: make
         { 255 0 175 }
     } [ { 0 0 255 } distance ] infimum-by
 ] unit-test
-

core/sequences/sequences.factor

@@ -104,7 +104,10 @@ INSTANCE: f immutable-sequence
 ! Integer sequences
 TUPLE: iota { n integer read-only } ;
 
-: iota ( n -- iota ) \ iota boa ; inline
+ERROR: non-negative-integer-expected n ;
+
+: iota ( n -- iota )
+    dup 0 < [ non-negative-integer-expected ] when \ iota boa ; inline
 
 M: iota length n>> ; inline
 M: iota nth-unsafe drop ; inline