Support for .?[] operator on openArrays (#52)

* Support for .?[] operator on openArrays

* Operator .?[] evaluates openArray expression only once

* Fix for Nim 1.2.x

---------

Co-authored-by: Mark Spanbroek <mark@spanbroek.net>

questionable/indexing.nim

@@ -1,16 +1,24 @@
 import std/macros
+import std/options
 
-macro `.?`*(expression: seq | string, brackets: untyped{nkBracket}): untyped =
-  # chain is of shape: (seq or string).?[index]
-  let index = brackets[0]
-  quote do:
-    block:
-      type T = typeof(`expression`[`index`])
-      let evaluated = `expression`
-      if `index` < evaluated.len:
-        evaluated[`index`].some
-      else:
-        T.none
+proc safeGet[T](expression: seq[T] | openArray[T], index: int): Option[T] =
+  if index >= expression.low and index <= expression.high:
+    expression[index].some
+  else:
+    T.none
+
+proc safeGet(expression: string, index: int): Option[char] =
+  if index >= expression.low and index <= expression.high:
+    expression[index].some
+  else:
+    char.none
+
+macro `.?`*(expression: seq | string | openArray, brackets: untyped{nkBracket}): untyped =
+    # chain is of shape: (seq or string or openArray).?[index]
+    let index = brackets[0]
+    quote do:
+      block:
+        safeGet(`expression`, `index`)
 
 macro `.?`*(expression: typed, brackets: untyped{nkBracket}): untyped =
   # chain is of shape: expression.?[index]

testmodules/options/test.nim

@@ -337,13 +337,28 @@ suite "optionals":
 
   test ".?[] can be used for indexing strings without raising IndexDefect":
     let str = "a"
-    check str.?[0] == 'a'.some 
+    check str.?[0] == 'a'.some
     check str.?[1] == char.none
+    check str.?[-1] == char.none
 
   test ".?[] can be used for indexing sequences without raising IndexDefect":
     let sequence = @[1]
     check sequence.?[0] == 1.some
     check sequence.?[1] == int.none
+    check sequence.?[-1] == int.none
+
+  test ".?[] can be used for indexing openArrays without raising IndexDefect":
+    proc checkOpenArray(oa: openArray[int]): void =
+      check oa.?[0] == 1.some
+      check oa.?[1] == int.none
+      check oa.?[-1] == int.none
+
+    checkOpenArray(@[1])
+
+  test ".?[] evaluates openArray expression only once":
+    var count = 0
+    discard (inc count; @[1].toOpenArray(0, 0)).?[0]
+    check count == 1
 
   test ".?[] can be followed by calls, operators and indexing":
     let table = @{"a": @[41, 42]}.toTable