scripts: csv.py: Adopted square brackets ([]) for by exprs

By fields are very different from normal fields in exprs (no type
checking, restricted subexprs, etc), so it makes sense to give them
separate syntaxes to clarify this distinction and improve readability.

This commit adopts optional square brackets for by fields, mimicking
generic/template specialization found in other languages:

Before:

  -fx=enumerate()
  -fy=enumerate(a,b)
  -fz=accumulate(z,a,b)

After:

  -fx=enumerate()
  -fy=enumerate[a,b]()
  -fz=accumulate[a,b](z)

Hopefully the readability argument is pretty obvious.

I went with square brackets to avoid parser ambiguities with <>. To be
honest I've never understood why C++ went with <>, array/function
confusion seems easier to resolve than ambiguous binary/index syntaxes,
but what do I know.

scripts/csv.py

@@ -654,7 +654,12 @@ class CsvExpr:
 
     # expr node base class
     class Expr:
-        def __init__(self, *args):
+        def __init__(self, *args, by=None):
+            self.by = by or []
+            for k in self.by:
+                if not isinstance(k, CsvExpr.Field):
+                    raise CsvExpr.Error("complicated by? %s" % k)
+
             for k, v in zip('abcdefghijklmnopqrstuvwxyz', args):
                 setattr(self, k, v)
 
@@ -668,8 +673,10 @@ class CsvExpr:
             return sum(1 for _ in self)
 
         def __repr__(self):
-            return '%s(%s)' % (
+            return '%s%s(%s)' % (
                     self.__class__.__name__,
+                    '[%s]' % (','.join(repr(k) for k in self.by))
+                        if self.by else '',
                     ','.join(repr(v) for v in self))
 
         def fields(self):
@@ -741,9 +748,14 @@ class CsvExpr:
 
     # func expr helper
     def func(funcs):
-        def func(name, args="a"):
+        def func(name, by=None, args=None):
+            if by is None and args is None:
+                by, args = None, "a"
+            elif args is None:
+                by, args = None, by
             def func(f):
                 f._func = name
+                f._fby = by
                 f._fargs = args
                 funcs[f._func] = f
                 return f
@@ -960,22 +972,16 @@ class CsvExpr:
                 return CsvGStddev()([v.eval(fields, state) for v in self])
 
     # enumerate exprs
-    @func('enumerate', '[*by]')
+    @func('enumerate', 'by', '')
     class Enumerate(Expr):
         """A [per by] number incremented for each result"""
         def fields(self):
-            # don't typecheck by fields
             return set()
 
         def type(self, types={}):
-            # don't typecheck, but make sure we can read by fields
-            for v in self:
-                if not isinstance(v, CsvExpr.Field):
-                    raise CsvExpr.Error("complicated by field? %s" % v)
             return CsvInt
 
         def fold(self, types={}):
-            # don't typecheck by fields
             return CsvSum
 
         def eval(self, fields={}, state=None):
@@ -984,7 +990,7 @@ class CsvExpr:
 
             # enumerate
             k = ('enumerate', id(self)) + tuple(
-                    fields.get(v.a) for v in self)
+                    fields.get(k.a) for k in self.by)
             x = state.get(k)
             if x is None:
                 z = 0
@@ -994,25 +1000,9 @@ class CsvExpr:
             state[k] = z
             return CsvInt(z)
 
-    @func('accumulate', 'a[, *by]')
+    @func('accumulate', 'by', 'a')
     class Accumulate(Expr):
         """A [per by] running sum across results"""
-        def fields(self):
-            # don't typecheck by fields
-            return self.a.fields()
-
-        def type(self, types={}):
-            # don't typecheck, but make sure we can read by fields
-            t = self.a.type(types)
-            for v in it.islice(self, 1, None):
-                if not isinstance(v, CsvExpr.Field):
-                    raise CsvExpr.Error("complicated by field? %s" % v)
-            return t
-
-        def fold(self, types={}):
-            # don't typecheck by fields
-            return self.a.fold(types)
-
         def eval(self, fields={}, state=None):
             y = self.a.eval(fields, state)
             if state is None:
@@ -1020,7 +1010,7 @@ class CsvExpr:
 
             # accumulate
             k = ('accumulate', id(self)) + tuple(
-                    fields.get(v.a) for v in it.islice(self, 1, None))
+                    fields.get(k.a) for k in self.by)
             x = state.get(k)
             if x is None:
                 z = y
@@ -1030,25 +1020,9 @@ class CsvExpr:
             state[k] = z
             return z
 
-    @func('delta', 'a[, *by]')
+    @func('delta', 'by', 'a')
     class Delta(Expr):
         """A [per by] difference between subsequent results"""
-        def fields(self):
-            # don't typecheck by fields
-            return self.a.fields()
-
-        def type(self, types={}):
-            # don't typecheck, but make sure we can read by fields
-            t = self.a.type(types)
-            for v in it.islice(self, 1, None):
-                if not isinstance(v, CsvExpr.Field):
-                    raise CsvExpr.Error("complicated by field? %s" % v)
-            return t
-
-        def fold(self, types={}):
-            # don't typecheck by fields
-            return self.a.fold(types)
-
         def eval(self, fields={}, state=None):
             y = self.a.eval(fields, state)
             if state is None:
@@ -1056,7 +1030,7 @@ class CsvExpr:
 
             # compute delta
             k = ('delta', id(self)) + tuple(
-                    fields.get(v.a) for v in it.islice(self, 1, None))
+                    fields.get(k.a) for k in self.by)
             x = state.get(k)
             if x is None:
                 z = y
@@ -1531,7 +1505,12 @@ class CsvExpr:
         print('funcs:')
         for func in cls.funcs.keys():
             print('  %-21s %s' % (
-                    '%s(%s)' % (func, CsvExpr.funcs[func]._fargs),
+                    '%s%s(%s)' % (
+                        func,
+                        '[%s]' % CsvExpr.funcs[func]._fby
+                            if CsvExpr.funcs[func]._fby
+                            else '',
+                        CsvExpr.funcs[func]._fargs),
                     CsvExpr.funcs[func].__doc__))
 
     # parse an expr
@@ -1560,6 +1539,25 @@ class CsvExpr:
             elif p.match('[_a-zA-Z][_a-zA-Z0-9]*'):
                 a = p.chomp()
 
+                by = None
+                if p.match('\['):
+                    p.chomp()
+                    if a not in CsvExpr.funcs:
+                        raise CsvExpr.Error("unknown function? %s" % a)
+                    by = []
+                    while True:
+                        if not p.match('\]'):
+                            b = p_expr(p)
+                            by.append(b)
+                            if p.match(','):
+                                p.chomp()
+                                continue
+                        if not p.match('\]'):
+                            raise CsvExpr.Error("mismatched squares? %s" % p)
+                        p.chomp()
+                        break
+
+                args = None
                 if p.match('\('):
                     p.chomp()
                     if a not in CsvExpr.funcs:
@@ -1575,8 +1573,12 @@ class CsvExpr:
                         if not p.match('\)'):
                             raise CsvExpr.Error("mismatched parens? %s" % p)
                         p.chomp()
-                        a = CsvExpr.funcs[a](*args)
                         break
+
+                if args is not None:
+                    a = CsvExpr.funcs[a](*args, by=by)
+                elif by is not None:
+                    raise CsvExpr.Error("expected parens? %s" % p)
                 else:
                     a = CsvExpr.Field(a)