[Python-ideas] A "local" pseudo-function

Greg Ewing greg.ewing at canterbury.ac.nz
Tue May 1 01:49:38 EDT 2018

Tim Peters wrote:
> I expected the following would work, but it doesn't :-)
>     iseven = lambda n: (
>                lambda n=n, \
>                       even = (lambda n: n == 0 or odd(n-1)), \
>                       odd = (lambda n: False if n == 0 else even(n-1)):
>                    even(n))()
> Ugly and obscure, but why not?  In the inner lambda, `n`, `even`, and
> `odd` are all defined in its namespace,

But 'even' and 'odd' not defined in the environment of the lambdas
assigned to them, because default values of a function's arguments
are evaluated outside of that function.

> For `even` to know at compile-time that `odd` will show up later in
> its enclosing lambda's arglist requires that Python do `letrec`-style
> binding instead.  For a start ;-)

One could envisage adding a letrec-like construct, but making the
argument list of an ordinary lambda behave like a letrec would be
warping things rather too much, IMO.

Personally I'd rather just add a "where" clause, and backwards
compatibility be damned. :-)


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