Add templateutils.evalTemplateParamOnce

stew/templateutils.nim

@@ -0,0 +1,53 @@
+type CppVar[T] = distinct ptr T
+
+iterator evalTemplateParamOnceImpl[T](x: T): lent T =
+  yield x
+
+when defined(cpp):
+  # TODO `nim cpp` miscompiles iterators returning `var`,
+  #      so we need to emulate them in terms of pointers:
+  iterator evalTemplateParamOnceImpl[T](x: var T): CppVar[T] =
+    yield CppVar[T](addr(x))
+
+  template stripCppVar[T](p: CppVar[T]): var T =
+    ((ptr T)(p))[]
+else:
+  iterator evalTemplateParamOnceImpl[T](x: var T): var T =
+    yield x
+
+template evalTemplateParamOnce*(templateParam, newName, blk: untyped) =
+  ## This can be used in templates to avoid the problem of multiple
+  ## evaluation of template parameters. Compared to the naive approach
+  ## of introducing an additional local variable, it has two benefits:
+  ##
+  ## * It avoids copying whenever possible.
+  ## * It works for var parameters.
+  ##
+  ## Usage example:
+  ##
+  ## template foo(xParam: SomeType) =
+  ##   evalTemplateParamOnce(xParam, x):
+  ##     echo x
+  ##     echo x
+  ##
+  ##  A currently existing limitation is that the `evalTemplateParamOnce`
+  ##  block is considered a `void` expression, so templates returning
+  ##  expressions may find it difficult to benefit fully from the construct.
+  ##
+  ##  Please also note that using conrol-flow statements such as `return`,
+  ##  `continue` and `break` within the template code is possible, but
+  ##  extra care must be taken to ensure that they are not referring to the
+  ##  inserted `for` loop (you may need to introduce enclosing named blocks
+  ##  for correct implementation of both `break` and `continue`).
+  ##
+  ##  Both limitations will be lifted in a future implementation based on
+  ##  view types.
+  block:
+    for paramAddr in evalTemplateParamOnceImpl(templateParam):
+      template newName: auto =
+        when paramAddr is CppVar:
+          stripCppVar(paramAddr)
+        else:
+          paramAddr
+
+      blk

tests/all_tests.nim

@@ -27,6 +27,7 @@ import
   test_ptrops,
   test_sequtils2,
   test_sorted_set,
+  test_templateutils,
   test_results,
   test_varints,
   test_winacl

tests/test_templateutils.nim

@@ -0,0 +1,64 @@
+import
+  unittest,
+  ../stew/templateutils
+
+var computations = newSeq[string]()
+var templateParamAddresses = newSeq[pointer]()
+
+type
+  ObjectHoldingSeq = object
+    data: seq[int]
+
+proc accessSeq(x: var ObjectHoldingSeq): var seq[int] =
+  computations.add("accessor")
+  x.data
+
+proc expensiveComputation(evaluationLabel: string): seq[int] =
+  computations.add evaluationLabel
+  return @[1, 2, 3]
+
+template reject(code: untyped) =
+  static: assert(not compiles(code))
+
+template evalManyTimes(xParam: untyped, shouldBeMutable: bool): string =
+  var res: string
+  evalTemplateParamOnce(xParam, x):
+    res = $x
+
+    when shouldBeMutable:
+      x.add 10
+    else:
+      reject:
+        x.add(10)
+
+    res.add " => "
+    res.add $x
+
+    templateParamAddresses.add(unsafeAddr x)
+  res
+
+test "Template utils":
+  # Pass function call
+  check "@[1, 2, 3] => @[1, 2, 3]" == evalManyTimes(
+    expensiveComputation("call"), shouldBeMutable = false)
+
+  # Pass var symbol
+  var s1 = expensiveComputation("var")
+  check "@[1, 2, 3] => @[1, 2, 3, 10]" == evalManyTimes(
+    s1, shouldBeMutable = true)
+
+  # Pass let symbol:
+  let s2 = expensiveComputation("let")
+  check "@[1, 2, 3] => @[1, 2, 3]" == evalManyTimes(
+    s2, shouldBeMutable = false)
+
+  var o = ObjectHoldingSeq(data: @[1, 2, 3])
+  check "@[1, 2, 3] => @[1, 2, 3, 10]" == evalManyTimes(
+    o.accessSeq, shouldBeMutable = true)
+
+  check computations == ["call", "var", "let", "accessor"]
+
+  check:
+    templateParamAddresses[1] == addr s1
+    templateParamAddresses[2] == unsafeAddr s2
+    templateParamAddresses[3] == addr o.accessSeq