From e99298d8719c71ff23921fdb9a79197880ac5bfc Mon Sep 17 00:00:00 2001
From: Tom Smeding <tom.smeding@gmail.com>
Date: Thu, 21 Nov 2019 23:23:38 +0100
Subject: Rotate arguments to substr and allow negative length

---
 VM.hs                 | 5 +++--
 tests/stringtest.lisp | 2 +-
 tests/stringtest.out  | 2 +-
 3 files changed, 5 insertions(+), 4 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
diff --git a/tests/stringtest.lisp b/tests/stringtest.lisp
index aa4898c..57a7ec4 100644
--- a/tests/stringtest.lisp
+++ b/tests/stringtest.lisp
@@ -4,6 +4,6 @@
 
 (for 0 (length str) (lambda (i)
     (if (= (mod i 60) 0)
-        (sys-put-string stdout (concat "<" (substr str i 10) ">"))
+        (sys-put-string stdout (concat "<" (substr i 10 str) ">"))
         '())))
 (print "")
diff --git a/tests/stringtest.out b/tests/stringtest.out
index a0c457b..5011462 100644
--- a/tests/stringtest.out
+++ b/tests/stringtest.out
@@ -1 +1 @@
-<#include "><test.lisp"><d i 60) 0)><r str i 10>
+<#include "><test.lisp"><d i 60) 0)><r i 10 str>
-- 
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