revset: add function to build dict of positional and keyword arguments

revset: add function to build dict of positional and keyword arguments

Keyword arguments will be convenient for functions that will take more than
one optional or boolean flags. For example,

  file(pattern[, subrepos=false])
  subrepo([[pattern], status])

Because I don't think all functions should accept key=value syntax, getkwargs()
does not support variadic functions such as 'ancestor(*changeset)'.

The core logic is placed in the parser module because keyword arguments will
be more useful in the templater, where functions take more options. Test cases
will be added by the next patch.

--- a/mercurial/parser.py	Sat Jun 27 17:05:28 2015 +0900
+++ b/mercurial/parser.py	Sat Jun 27 17:25:01 2015 +0900
@@ -91,6 +91,38 @@
             return self.eval(t)
         return t
 
+def buildargsdict(trees, funcname, keys, keyvaluenode, keynode):
+    """Build dict from list containing positional and keyword arguments
+
+    Invalid keywords or too many positional arguments are rejected, but
+    missing arguments are just omitted.
+    """
+    if len(trees) > len(keys):
+        raise error.ParseError(_("%(func)s takes at most %(nargs)d arguments")
+                               % {'func': funcname, 'nargs': len(keys)})
+    args = {}
+    # consume positional arguments
+    for k, x in zip(keys, trees):
+        if x[0] == keyvaluenode:
+            break
+        args[k] = x
+    # remainder should be keyword arguments
+    for x in trees[len(args):]:
+        if x[0] != keyvaluenode or x[1][0] != keynode:
+            raise error.ParseError(_("%(func)s got an invalid argument")
+                                   % {'func': funcname})
+        k = x[1][1]
+        if k not in keys:
+            raise error.ParseError(_("%(func)s got an unexpected keyword "
+                                     "argument '%(key)s'")
+                                   % {'func': funcname, 'key': k})
+        if k in args:
+            raise error.ParseError(_("%(func)s got multiple values for keyword "
+                                     "argument '%(key)s'")
+                                   % {'func': funcname, 'key': k})
+        args[k] = x[2]
+    return args
+
 def _prettyformat(tree, leafnodes, level, lines):
     if not isinstance(tree, tuple) or tree[0] in leafnodes:
         lines.append((level, str(tree)))

--- a/mercurial/revset.py	Sat Jun 27 17:05:28 2015 +0900
+++ b/mercurial/revset.py	Sat Jun 27 17:25:01 2015 +0900
@@ -282,6 +282,10 @@
         raise error.ParseError(err)
     return l
 
+def getkwargs(x, funcname, keys):
+    return parser.buildargsdict(getlist(x), funcname, keys.split(),
+                                keyvaluenode='keyvalue', keynode='symbol')
+
 def isvalidsymbol(tree):
     """Examine whether specified ``tree`` is valid ``symbol`` or not
     """