add ?subseq, ?tail*, ?head*

extra/sequences/lib/lib-tests.factor

@@ -20,8 +20,6 @@ IN: temporary
 [ 10 ] [ { 1 2 3 4 } [ + ] reduce* ] unit-test
 [ 24 ] [ { 1 2 3 4 } [ * ] reduce* ] unit-test
 
-[ 1 2 3 4 ] [ { 1 2 3 4 } 4 nfirst ] unit-test
-
 [ -4 ] [ 1 -4 [ abs ] higher ] unit-test
 [ 1 ] [ 1 -4 [ abs ] lower ] unit-test
 
@@ -80,4 +78,4 @@ IN: temporary
 { 13 } [ 1 2 { 3 4 } [ + + ] 2 each-withn + ] unit-test
 [ { 910 911 912 } ] [ 10 900 3 [ + + ] map-with2 ] unit-test
 
-[ 1 2 3 4 ] [ { 1 2 3 4 } 4 nfirst ] unit-test
+[ 1 2 3 4 ] [ { 1 2 3 4 } 4 firstn ] unit-test

extra/sequences/lib/lib.factor

@@ -18,8 +18,9 @@ IN: sequences.lib
 
 : map-with2 ( obj obj list quot -- newseq ) 2 map-withn ; inline
 
-MACRO: nfirst ( n -- )
-    [ [ swap nth ] curry [ keep ] curry ] map concat [ drop ] compose ;
+MACRO: firstn ( n -- )
+    [ [ swap nth ] curry
+    [ keep ] curry ] map concat [ drop ] compose ;
 
 : prepare-index ( seq quot -- seq n quot )
     >r dup length r> ; inline
@@ -182,6 +183,14 @@ PRIVATE>
 : ?first3 ( seq -- 1st/f 2nd/f 3rd/f ) dup ?first2 rot ?third ; inline
 : ?first4 ( seq -- 1st/f 2nd/f 3rd/f 4th/f ) dup ?first3 roll ?fourth ; inline
 
+USE: continuations
+: ?subseq ( from to seq -- subseq )
+    >r >r 0 max r> r>
+    [ length tuck min >r min r> ] keep subseq ;
+
+: ?head* ( seq n -- seq/f ) (head) ?subseq ;
+: ?tail* ( seq n -- seq/f ) (tail) ?subseq ;
+
 : accumulator ( quot -- quot vec )
     V{ } clone [ [ push ] curry compose ] keep ;