Add mitems and mpairs where it makes sense

lib/system.nim

@@ -1653,6 +1653,13 @@ iterator items*[T](a: openArray[T]): T {.inline.} =
     yield a[i]
     inc(i)
 
+iterator mitems*[T](a: var openArray[T]): var T {.inline.} =
+  ## iterates over each item of `a` so that you can modify the yielded value.
+  var i = 0
+  while i < len(a):
+    yield a[i]
+    inc(i)
+
 iterator items*[IX, T](a: array[IX, T]): T {.inline.} =
   ## iterates over each item of `a`.
   var i = low(IX)
@@ -1662,6 +1669,15 @@ iterator items*[IX, T](a: array[IX, T]): T {.inline.} =
       if i >= high(IX): break
       inc(i)
 
+iterator mitems*[IX, T](a: var array[IX, T]): var T {.inline.} =
+  ## iterates over each item of `a` so that you can modify the yielded value.
+  var i = low(IX)
+  if i <= high(IX):
+    while true:
+      yield a[i]
+      if i >= high(IX): break
+      inc(i)
+
 iterator items*[T](a: set[T]): T {.inline.} =
   ## iterates over each element of `a`. `items` iterates only over the
   ## elements that are really in the set (and not over the ones the set is
@@ -1692,6 +1708,14 @@ iterator pairs*[T](a: openArray[T]): tuple[key: int, val: T] {.inline.} =
     yield (i, a[i])
     inc(i)
 
+iterator mpairs*[T](a: var openArray[T]): tuple[key: int, val: var T] {.inline.} =
+  ## iterates over each item of `a`. Yields ``(index, a[index])`` pairs.
+  ## ``a[index]`` can be modified.
+  var i = 0
+  while i < len(a):
+    yield (i, a[i])
+    inc(i)
+
 iterator pairs*[IX, T](a: array[IX, T]): tuple[key: IX, val: T] {.inline.} =
   ## iterates over each item of `a`. Yields ``(index, a[index])`` pairs.
   var i = low(IX)
@@ -1701,6 +1725,16 @@ iterator pairs*[IX, T](a: array[IX, T]): tuple[key: IX, val: T] {.inline.} =
       if i >= high(IX): break
       inc(i)
 
+iterator mpairs*[IX, T](a: var array[IX, T]): tuple[key: IX, val: var T] {.inline.} =
+  ## iterates over each item of `a`. Yields ``(index, a[index])`` pairs.
+  ## ``a[index]`` can be modified.
+  var i = low(IX)
+  if i <= high(IX):
+    while true:
+      yield (i, a[i])
+      if i >= high(IX): break
+      inc(i)
+
 iterator pairs*[T](a: seq[T]): tuple[key: int, val: T] {.inline.} =
   ## iterates over each item of `a`. Yields ``(index, a[index])`` pairs.
   var i = 0
@@ -1708,6 +1742,14 @@ iterator pairs*[T](a: seq[T]): tuple[key: int, val: T] {.inline.} =
     yield (i, a[i])
     inc(i)
 
+iterator mpairs*[T](a: var seq[T]): tuple[key: int, val: var T] {.inline.} =
+  ## iterates over each item of `a`. Yields ``(index, a[index])`` pairs.
+  ## ``a[index]`` can be modified.
+  var i = 0
+  while i < len(a):
+    yield (i, a[i])
+    inc(i)
+
 iterator pairs*(a: string): tuple[key: int, val: char] {.inline.} =
   ## iterates over each item of `a`. Yields ``(index, a[index])`` pairs.
   var i = 0
@@ -1715,6 +1757,14 @@ iterator pairs*(a: string): tuple[key: int, val: char] {.inline.} =
     yield (i, a[i])
     inc(i)
 
+iterator mpairs*(a: var string): tuple[key: int, val: var char] {.inline.} =
+  ## iterates over each item of `a`. Yields ``(index, a[index])`` pairs.
+  ## ``a[index]`` can be modified.
+  var i = 0
+  while i < len(a):
+    yield (i, a[i])
+    inc(i)
+
 
 proc isNil*[T](x: seq[T]): bool {.noSideEffect, magic: "IsNil".}
 proc isNil*[T](x: ref T): bool {.noSideEffect, magic: "IsNil".}
@@ -2992,6 +3042,15 @@ iterator items*[T](a: seq[T]): T {.inline.} =
     inc(i)
     assert(len(a) == L, "seq modified while iterating over it")
 
+iterator mitems*[T](a: var seq[T]): var T {.inline.} =
+  ## iterates over each item of `a` so that you can modify the yielded value.
+  var i = 0
+  let L = len(a)
+  while i < L:
+    yield a[i]
+    inc(i)
+    assert(len(a) == L, "seq modified while iterating over it")
+
 iterator items*(a: string): char {.inline.} =
   ## iterates over each item of `a`.
   var i = 0
@@ -3001,6 +3060,15 @@ iterator items*(a: string): char {.inline.} =
     inc(i)
     assert(len(a) == L, "string modified while iterating over it")
 
+iterator mitems*(a: var string): var char {.inline.} =
+  ## iterates over each item of `a` so that you can modify the yielded value.
+  var i = 0
+  let L = len(a)
+  while i < L:
+    yield a[i]
+    inc(i)
+    assert(len(a) == L, "string modified while iterating over it")
+
 when not defined(nimhygiene):
   {.pragma: inject.}