Adds convenience mapIt templates.

lib/pure/collections/sequtils.nim

@@ -276,6 +276,38 @@ template foldr*(sequence, operation: expr): expr =
     result = operation
   result
 
+template mapIt*(seq1, typ, pred: expr): expr =
+  ## Convenience template around the ``map`` proc to reduce typing.
+  ##
+  ## The template injects the ``it`` variable which you can use directly in an
+  ## expression. You also need to pass as `typ` the type of the expression,
+  ## since the new returned sequence can have a different type than the
+  ## original.  Example:
+  ##
+  ## .. code-block:: nimrod
+  ##   let
+  ##     nums = @[1, 2, 3, 4]
+  ##     strings = nums.mapIt(string, $(4 * it))
+  var result {.gensym.}: seq[typ] = @[]
+  for it {.inject.} in items(seq1):
+    result.add(pred)
+  result
+
+template mapIt*(varSeq, pred: expr) =
+  ## Convenience template around the mutable ``map`` proc to reduce typing.
+  ##
+  ## The template injects the ``it`` variable which you can use directly in an
+  ## expression. The expression has to return the same type as the sequence you
+  ## are mutating. Example:
+  ##
+  ## .. code-block:: nimrod
+  ##   var nums = @[1, 2, 3, 4]
+  ##   nums.mapIt(it * 3)
+  ##   assert nums[0] + nums[3] == 15
+  for i in 0 .. <len(varSeq):
+    let it {.inject.} = varSeq[i]
+    varSeq[i] = pred
+
 when isMainModule:
   import strutils
   block: # concat test
@@ -381,4 +413,11 @@ when isMainModule:
     Inserting [2,2,2,2,2,2] into [1,1,1,1,1,1,1,1]
     at 3 is [1,1,1,2,2,2,2,2,2,1,1,1,1,1]"""
 
+  block: # mapIt tests
+    var
+      nums = @[1, 2, 3, 4]
+      strings = nums.mapIt(string, $(4 * it))
+    nums.mapIt(it * 3)
+    assert nums[0] + nums[3] == 15
+
   echo "Finished doc tests"