sequences: adding ?last for symmetry.

basis/compiler/cfg/ssa/construction/construction.factor

@@ -92,7 +92,7 @@ SYMBOLS: stacks pushed ;
     if ;
 
 : (top-name) ( vreg -- vreg' )
-    stacks get at [ f ] [ last ] if-empty ;
+    stacks get at ?last ;
 
 : top-name ( vreg -- vreg' )
     (top-name)

basis/farkup/farkup.factor

@@ -150,9 +150,6 @@ DEFER: (parse-paragraph)
     rest
     dup last empty? [ but-last ] when ;
 
-: ?last ( seq -- elt/f )
-    [ f ] [ last ] if-empty ;
-
 : coalesce ( rows -- rows' )
     V{ } clone [
         '[

core/sequences/sequences-docs.factor

@@ -187,6 +187,10 @@ HELP: ?second
 { $values { "seq" sequence } { "elt/f" "an object or " { $link f } } }
 { $description "A forgiving version of " { $link second } ". If the sequence has less than two elements, or if the sequence is " { $link f } ", simply outputs " { $link f } "." } ;
 
+HELP: ?last
+{ $values { "seq" sequence } { "elt/f" "an object or " { $link f } } }
+{ $description "A forgiving version of " { $link last } ". If the sequence is empty, or if the sequence is " { $link f } ", simply outputs " { $link f } "." } ;
+
 HELP: nth-unsafe
 { $values { "n" "an integer" } { "seq" sequence } { "elt" object } }
 { $contract "Unsafe variant of " { $link nth } " that does not perform bounds checks." } ;

core/sequences/sequences-tests.factor

@@ -245,6 +245,10 @@ unit-test
 [ f ] [ { } ?first ] unit-test
 [ 0 ] [ 10 iota ?first ] unit-test
 
+[ f ] [ f ?last ] unit-test
+[ f ] [ { } ?last ] unit-test
+[ 9 ] [ 10 iota ?last ] unit-test
+
 [ -1/0. 0 remove-nth! ] must-fail
 [ "" ] [ "" [ CHAR: \s = ] trim ] unit-test
 [ "" ] [ "" [ CHAR: \s = ] trim-head ] unit-test

core/sequences/sequences.factor

@@ -177,6 +177,7 @@ PRIVATE>
 
 : ?first ( seq -- elt/f ) 0 swap ?nth ; inline
 : ?second ( seq -- elt/f ) 1 swap ?nth ; inline
+: ?last ( seq -- elt/f ) [ length 1 - ] [ ?nth ] bi ; inline
 
 MIXIN: virtual-sequence
 GENERIC: virtual-exemplar ( seq -- seq' )