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> The essential message is to make illegal states unrepresentable.

That's why, in C we have:

    if (a = b)

and:

    if (a < b < c)

that cause (in some compilers) warnings. I wanted to make this an error in D. One option is to check for and disallow them after the parse, but that left:

    if ((a < b) < c)

as still an error. I wanted to allow these constructs to remain if they were parenthesized, as that would imply they were deliberate. (Yes, parenthesized expressions could be a separate grammar, but I thought that was an awful idea.)

The solution is to fix the grammar, so they wouldn't even parse. The beauty of this is one doesn't have to enable warnings, or have a separate check for them, it just won't compile. Even better, D's expression grammar is the same as C's, and it did not upset anything else in the grammar as a side effect.

Based on my experience at Boeing designing hardware, defining the problem out of existence is a great way to design robust systems.

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In [1]: import dis In [2]: def a(): ...: return (1 in [1,2,3] is True) In [3]: def b(): ..: return ((1 in [1,2,3]) is True) In [4]: a() Out[4]: False In [5]: b() Out[5]: True In [6]: dis.dis(a) 2 0 LOAD_CONST 1 (1) 2 LOAD_CONST 1 (1) 4 LOAD_CONST 2 (2) 6 LOAD_CONST 3 (3) 8 BUILD_LIST 3 10 DUP_TOP 12 ROT_THREE 14 COMPARE_OP 6 (in) 16 JUMP_IF_FALSE_OR_POP 24 18 LOAD_CONST 4 (True) 20 COMPARE_OP 8 (is) 22 RETURN_VALUE >> 24 ROT_TWO 26 POP_TOP 28 RETURN_VALUE In [7]: dis.dis(b) 2 0 LOAD_CONST 1 (1) 2 LOAD_CONST 5 ((1, 2, 3)) 4 COMPARE_OP 6 (in) 6 LOAD_CONST 4 (True) 8 COMPARE_OP 8 (is) 10 RETURN_VALUE

In [1]: import ast In [2]: print(ast.dump(ast.parse("""\ ...: def a(): ...: return (1 in [1,2,3] is True)"""))) Module( body=[ FunctionDef( name='a', args=arguments(args=[], vararg=None, kwonlyargs=[], kw_defaults=[], kwarg=None, defaults=[]), body=[ Return( value=Compare( left=Num(n=1), ops=[In(), Is()], comparators=[ List(elts=[Num(n=1), Num(n=2), Num(n=3)], ctx=Load()), NameConstant(value=True) ] ) ) ], decorator_list=[], returns=None ) ] ) In [3]: print(ast.dump(ast.parse("""\ ...: def b(): ...: return ((1 in [1,2,3]) is True)"""))) Module( body=[ FunctionDef( name='b', args=arguments(args=[], vararg=None, kwonlyargs=[], kw_defaults=[], kwarg=None, defaults=[]), body=[ Return( value=Compare( left=Compare( left=Num(n=1), ops=[In()], comparators=[List(elts=[Num(n=1), Num(n=2), Num(n=3)], ctx=Load())] ), ops=[Is()], comparators=[NameConstant(value=True)] ) ) ], decorator_list=[], returns=None ) ] )