Rotate arguments to substr and allow negative length

1 files changed, 3 insertions, 2 deletions

diff --git a/VM.hs b/VM.hs
index 64674fb..ea94597 100644
--- a/VM.hs
+++ b/VM.hs
@@ -152,8 +152,9 @@ vmRunBuiltin state "sys-stdin" [] = return (RVNum (-1), state)
 vmRunBuiltin state "sys-stdout" [] = return (RVNum (-2), state)
 vmRunBuiltin state "sys-stderr" [] = return (RVNum (-3), state)
 vmRunBuiltin state "length" [RVString str] = return (RVNum (length str), state)
-vmRunBuiltin state "substr" [RVString str, RVNum idx, RVNum len] =
-    return (RVString (take len (drop idx str)), state)
+vmRunBuiltin state "substr" [RVNum idx, RVNum len, RVString str] =
+    let s = (if len >= 0 then take len else id) (drop idx str)
+    in return (RVString s, state)
 vmRunBuiltin state "ord" [RVString str] = return (RVNum (case str of { "" -> 0; c:_ -> ord c }), state)
 vmRunBuiltin state "chr" [RVNum num] = return (RVString [chr num], state)
 vmRunBuiltin state "concat" values