Re: Lisp history: IF, etc.

On 2024-04-04, Lawrence D'Oliveiro <ldo@nz.invalid> wrote:

On Wed, 03 Apr 2024 15:29:52 -0700, Paul Rubin wrote:
>

I think lazy evaluation is not needed since the
input to the Church boolean (I hope I have the jargon right) is two
lambda abstractions.  The boolean selects one of them, and the result is
applied to whatever the next thing is.

>
I use this in Python, to try to avoid that abortion that is the Python
conditional expression. Instead of, say,

Category error: "that abortion" is Python. If you're in the middle of it,
you've already lost the ideological battle. Use it like it was meant to be.

   a = x / y if y != 0 else 0
>
I would do
>
    a = (lambda : 0, lambda : x / y)[y != 0]()

Not only is it more verbose, but it's unlikely that Python's compiler
recognizes the idiom and optimizes away the lambdas.

import dis

def f0():
  return x / y if y != 0 else 0

def f1():
  (lambda : 0, lambda : x / y)[y != 0]()

print('disasembly of f0')
dis.dis(f0)
print('disasembly of f1')
dis.dis(f1)

disasembly of f0
  4           0 LOAD_GLOBAL              0 (y)
              2 LOAD_CONST               1 (0)
              4 COMPARE_OP               3 (!=)
              6 POP_JUMP_IF_FALSE       16
              8 LOAD_GLOBAL              1 (x)
             10 LOAD_GLOBAL              0 (y)
             12 BINARY_TRUE_DIVIDE
             14 RETURN_VALUE
        >>   16 LOAD_CONST               1 (0)
             18 RETURN_VALUE
disasembly of f1
  7           0 LOAD_CONST               1 (<code object <lambda> at 0xb77c88b8, file "distest.py", line 7>)
              2 LOAD_CONST               2 ('f1.<locals>.<lambda>')
              4 MAKE_FUNCTION            0
              6 LOAD_CONST               3 (<code object <lambda> at 0xb77c8910, file "distest.py", line 7>)
              8 LOAD_CONST               2 ('f1.<locals>.<lambda>')
             10 MAKE_FUNCTION            0
             12 BUILD_TUPLE              2
             14 LOAD_GLOBAL              0 (y)
             16 LOAD_CONST               4 (0)
             18 COMPARE_OP               3 (!=)
             20 BINARY_SUBSCR
             22 CALL_FUNCTION            0
             24 POP_TOP
             26 LOAD_CONST               0 (None)
             28 RETURN_VALUE

Deliberately writing longer, harder-to-read code, that is also slower, takes a
special sort of cretin.

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