Merge pull request #1443 from def-/future-listcomprehensions

Add list comprehensions to future module

lib/pure/future.nim

@@ -115,3 +115,60 @@ macro `->`*(p, b: expr): expr {.immediate.} =
   ##     f(2, 2)
 
   result = createProcType(p, b)
+
+type ListComprehension = object
+var lc*: ListComprehension
+
+macro `[]`*(lc: ListComprehension, comp, typ: expr): expr =
+  ## List comprehension, returns a sequence. `comp` is the actual list
+  ## comprehension, for example ``x | (x <- 1..10, x mod 2 == 0)``. `typ` is
+  ## the type that will be stored inside the result seq.
+  ##
+  ## .. code-block:: nimrod
+  ##
+  ##   echo lc[x | (x <- 1..10, x mod 2 == 0), int]
+  ##
+  ##   const n = 20
+  ##   echo lc[(x,y,z) | (x <- 1..n, y <- x..n, z <- y..n, x*x + y*y == z*z),
+  ##           tuple[a,b,c: int]]
+
+  expectLen(comp, 3)
+  expectKind(comp, nnkInfix)
+  expectKind(comp[0], nnkIdent)
+  assert($comp[0].ident == "|")
+
+  result = newCall(
+    newDotExpr(
+      newIdentNode("result"),
+      newIdentNode("add")),
+    comp[1])
+
+  for i in countdown(comp[2].len-1, 0):
+    let x = comp[2][i]
+    expectKind(x, nnkInfix)
+    expectMinLen(x, 1)
+    if x[0].kind == nnkIdent and $x[0].ident == "<-":
+      expectLen(x, 3)
+      result = newNimNode(nnkForStmt).add(x[1], x[2], result)
+    else:
+      result = newIfStmt((x, result))
+
+  result = newNimNode(nnkCall).add(
+    newNimNode(nnkPar).add(
+      newNimNode(nnkLambda).add(
+        newEmptyNode(),
+        newEmptyNode(),
+        newEmptyNode(),
+        newNimNode(nnkFormalParams).add(
+          newNimNode(nnkBracketExpr).add(
+            newIdentNode("seq"),
+            typ)),
+        newEmptyNode(),
+        newEmptyNode(),
+        newStmtList(
+          newAssignment(
+            newIdentNode("result"),
+            newNimNode(nnkPrefix).add(
+              newIdentNode("@"),
+              newNimNode(nnkBracket))),
+          result))))