A motivating example for the new bindSym behavior.

The example is a proof-of-concept logging library, allowing you to define
lexically-scoped environments where certain logging attributes are applied
automatically to all logging statements.

fixes tmacro1 (use of `bindSym` inside static blocks)

tests/macros/tstructuredlogging.nim

@@ -0,0 +1,154 @@
+discard """
+output: '''
+main started: a=10, b=inner-b, c=10, d=some-d, x=16, z=20
+exiting: a=12, b=overriden-b, c=100, msg=bye bye, x=16
+'''
+"""
+
+import macros, tables
+
+template scopeHolder =
+  0 # scope revision number
+
+type
+  BindingsSet = Table[string, NimNode]
+
+proc actualBody(n: NimNode): NimNode =
+  # skip over the double StmtList node introduced in `mergeScopes`
+  result = n.body
+  if result.kind == nnkStmtList and result[0].kind == nnkStmtList:
+    result = result[0]
+
+iterator bindings(n: NimNode, skip = 0): (string, NimNode) =
+  for i in skip ..< n.len:
+    let child = n[i]
+    if child.kind in {nnkAsgn, nnkExprEqExpr}:
+      let name = $child[0]
+      let value = child[1]
+      yield (name, value)
+
+proc scopeRevision(scopeHolder: NimNode): int =
+  # get the revision number from a scopeHolder sym
+  assert scopeHolder.kind == nnkSym
+  var revisionNode = scopeHolder.getImpl.actualBody[0]
+  result = int(revisionNode.intVal)
+
+proc lastScopeHolder(scopeHolders: NimNode): NimNode =
+  # get the most recent scopeHolder from a symChoice node
+  if scopeHolders.kind in {nnkClosedSymChoice, nnkOpenSymChoice}:
+    var bestScopeRev = 0
+    assert scopeHolders.len > 0
+    for scope in scopeHolders:
+      let rev = scope.scopeRevision
+      if result == nil or rev > bestScopeRev:
+        result = scope
+        bestScopeRev = rev
+  else:
+    result = scopeHolders
+
+  assert result.kind == nnkSym
+
+macro mergeScopes(scopeHolders: typed, newBindings: untyped): untyped =
+  var
+    bestScope = scopeHolders.lastScopeHolder
+    bestScopeRev = bestScope.scopeRevision
+
+  var finalBindings = initTable[string, NimNode]()
+  for k, v in bindings(bestScope.getImpl.actualBody, skip = 1):
+    finalBindings[k] = v
+
+  for k, v in bindings(newBindings):
+    finalBindings[k] = v
+
+  var newScopeDefinition = newStmtList(newLit(bestScopeRev + 1))
+
+  for k, v in finalBindings:
+    newScopeDefinition.add newAssignment(newIdentNode(k), v)
+
+  result = quote:
+    template scopeHolder = `newScopeDefinition`
+
+template scope(newBindings: untyped) {.dirty.} =
+  mergeScopes(bindSym"scopeHolder", newBindings)
+
+type
+  TextLogRecord = object
+    line: string
+
+  StdoutLogRecord = object
+
+template setProperty(r: var TextLogRecord, key: string, val: string, isFirst: bool) =
+  if not first: r.line.add ", "
+  r.line.add key
+  r.line.add "="
+  r.line.add val
+
+template setEventName(r: var StdoutLogRecord, name: string) =
+  stdout.write(name & ": ")
+
+template setProperty(r: var StdoutLogRecord, key: string, val: auto, isFirst: bool) =
+  when not isFirst: stdout.write ", "
+  stdout.write key
+  stdout.write "="
+  stdout.write $val
+
+template flushRecord(r: var StdoutLogRecord) =
+  stdout.write "\n"
+  stdout.flushFile
+
+macro logImpl(scopeHolders: typed,
+              logStmtProps: varargs[untyped]): untyped =
+  let lexicalScope = scopeHolders.lastScopeHolder.getImpl.actualBody
+  var finalBindings = initOrderedTable[string, NimNode]()
+
+  for k, v in bindings(lexicalScope, skip = 1):
+    finalBindings[k] = v
+
+  for k, v in bindings(logStmtProps, skip = 1):
+    finalBindings[k] = v
+
+  finalBindings.sort(system.cmp)
+
+  let eventName = logStmtProps[0]
+  assert eventName.kind in {nnkStrLit}
+  let record = genSym(nskVar, "record")
+
+  result = quote:
+    var `record`: StdoutLogRecord
+    setEventName(`record`, `eventName`)
+
+  var isFirst = true
+  for k, v in finalBindings:
+    result.add newCall(newIdentNode"setProperty",
+                       record, newLit(k), v, newLit(isFirst))
+    isFirst = false
+
+  result.add newCall(newIdentNode"flushRecord", record)
+
+template log(props: varargs[untyped]) {.dirty.} =
+  logImpl(bindSym"scopeHolder", props)
+
+scope:
+  a = 12
+  b = "original-b"
+
+scope:
+  x = 16
+  b = "overriden-b"
+
+scope:
+  c = 100
+
+proc main =
+  scope:
+    c = 10
+
+  scope:
+    z = 20
+
+  log("main started", a = 10, b = "inner-b", d = "some-d")
+
+main()
+
+log("exiting", msg = "bye bye")
+