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Python syntax in Lisp and Scheme

Pascal Bourguignon spam at thalassa.informatimago.com
Wed Oct 15 23:16:17 EDT 2003 
Jacek Generowicz <jacek.generowicz at cern.ch> writes:

> mertz at gnosis.cx (David Mertz) writes:
> 
> > HOFs are not a special Lisp thing.  Haskell does them much better,
> > for example... and so does Python.
> 
> mertz at gnosis.cx (David Mertz) writes:
> 
> > the alpha and omega of HOFs is that functions are first class
> > objects that can be passed and returned.
> 
> How do you reconcile these two statements ?
> 
> [Hint: functions in Lisps are "first class objects that can be passed
> and returned"; how does Python (or Haskell) do this "alpha and omega
> of HOFs" "much better" ?]


Well, to be,  a first class object is an object  I can "process".  For
example, a list is a first class object:

[37]> (setq x '(a b c))
(A B C)
[42]> (setf (cadr x) 'd)
D
[43]> x
(A D C)


So, to be really a first class object, a function should let itself be
modified too:

[44]> (setq f (function (lambda (x) (+ 1 x))))
#<CLOSURE :LAMBDA (X) (+ 1 X)>

Oops, an opaque type, I can't change the body.

[46]> (setq f (lambda (x) (+ 1 x)))
#<CLOSURE :LAMBDA (X) (+ 1 X)>

No luck.

[47]> (setq f '(lambda (x) (+ 1 x)))
(LAMBDA (X) (+ 1 X))

Ok, ok, this is only a list...

[48]> (setf (second (caddr f)) 2)
2
[49]> f
(LAMBDA (X) (+ 2 X))
[50]> (funcall f 3)
*** - FUNCALL: argument (LAMBDA (X) (+ 2 X)) is not a function.

See, not a first class object...


[52]> (eval `(,f 3))
5

Ha!  My precious, precious eval...


-- 
__Pascal_Bourguignon__
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