Adds foldl and foldr templates to sequtils.

lib/pure/collections/sequtils.nim

@@ -151,6 +151,78 @@ template toSeq*(iter: expr): expr {.immediate.} =
   for x in iter: add(result, x)
   result
 
+template foldl*(sequence, operation: expr): expr =
+  ## Template to fold a sequence from left to right, returning the accumulation.
+  ##
+  ## The sequence is required to have at least a single element. Debug versions
+  ## of your program will assert in this situation but release versions will
+  ## happily go ahead. If the sequence has a single element it will be returned
+  ## without applying ``operation``.
+  ##
+  ## The ``operation`` parameter should be an expression which uses the
+  ## variables ``a`` and ``b`` for each step of the fold. Since this is a left
+  ## fold, for non associative binary operations like substraction think that
+  ## the sequence of numbers 1, 2 and 3 will be parenthesized as (((1) - 2) -
+  ## 3).  Example:
+  ##
+  ## .. code-block:: nimrod
+  ##   let
+  ##     numbers = @[5, 9, 11]
+  ##     addition = foldl(numbers, a + b)
+  ##     substraction = foldl(numbers, a - b)
+  ##     multiplication = foldl(numbers, a * b)
+  ##     words = @["nim", "rod", "is", "cool"]
+  ##     concatenation = foldl(words, a & b)
+  ##   assert addition == 25, "Addition is (((5)+9)+11)"
+  ##   assert substraction == -15, "Substraction is (((5)-9)-11)"
+  ##   assert multiplication == 495, "Multiplication is (((5)*9)*11)"
+  ##   assert concatenation == "nimrodiscool"
+  assert sequence.len > 0, "Can't fold empty sequences"
+  var result {.gensym.}: type(sequence[0])
+  result = sequence[0]
+  for i in countup(1, sequence.len - 1):
+    let
+      a {.inject.} = result
+      b {.inject.} = sequence[i]
+    result = operation
+  result
+
+template foldr*(sequence, operation: expr): expr =
+  ## Template to fold a sequence from right to left, returning the accumulation.
+  ##
+  ## The sequence is required to have at least a single element. Debug versions
+  ## of your program will assert in this situation but release versions will
+  ## happily go ahead. If the sequence has a single element it will be returned
+  ## without applying ``operation``.
+  ##
+  ## The ``operation`` parameter should be an expression which uses the
+  ## variables ``a`` and ``b`` for each step of the fold. Since this is a right
+  ## fold, for non associative binary operations like substraction think that
+  ## the sequence of numbers 1, 2 and 3 will be parenthesized as (1 - (2 -
+  ## (3))). Example:
+  ##
+  ## .. code-block:: nimrod
+  ##   let
+  ##     numbers = @[5, 9, 11]
+  ##     addition = foldr(numbers, a + b)
+  ##     substraction = foldr(numbers, a - b)
+  ##     multiplication = foldr(numbers, a * b)
+  ##     words = @["nim", "rod", "is", "cool"]
+  ##     concatenation = foldr(words, a & b)
+  ##   assert addition == 25, "Addition is (5+(9+(11)))"
+  ##   assert substraction == 7, "Substraction is (5-(9-(11)))"
+  ##   assert multiplication == 495, "Multiplication is (5*(9*(11)))"
+  ##   assert concatenation == "nimrodiscool"
+  assert sequence.len > 0, "Can't fold empty sequences"
+  var result {.gensym.}: type(sequence[0])
+  result = sequence[sequence.len - 1]
+  for i in countdown(sequence.len - 2, 0):
+    let
+      a {.inject.} = sequence[i]
+      b {.inject.} = result
+    result = operation
+  result
+
 when isMainModule:
   import strutils
   block: # concat test
@@ -210,4 +282,30 @@ when isMainModule:
           result = true)
     assert odd_numbers == @[1, 3, 5, 7, 9]
 
+  block: # foldl tests
+    let
+      numbers = @[5, 9, 11]
+      addition = foldl(numbers, a + b)
+      substraction = foldl(numbers, a - b)
+      multiplication = foldl(numbers, a * b)
+      words = @["nim", "rod", "is", "cool"]
+      concatenation = foldl(words, a & b)
+    assert addition == 25, "Addition is (((5)+9)+11)"
+    assert substraction == -15, "Substraction is (((5)-9)-11)"
+    assert multiplication == 495, "Multiplication is (((5)*9)*11)"
+    assert concatenation == "nimrodiscool"
+
+  block: # foldr tests
+    let
+      numbers = @[5, 9, 11]
+      addition = foldr(numbers, a + b)
+      substraction = foldr(numbers, a - b)
+      multiplication = foldr(numbers, a * b)
+      words = @["nim", "rod", "is", "cool"]
+      concatenation = foldr(words, a & b)
+    assert addition == 25, "Addition is (5+(9+(11)))"
+    assert substraction == 7, "Substraction is (5-(9-(11)))"
+    assert multiplication == 495, "Multiplication is (5*(9*(11)))"
+    assert concatenation == "nimrodiscool"
+
   echo "Finished doc tests"