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# Whoa! Do Python and Lisp really have LAMBDA ?

 P: n/a Earlier Ed Schofield (thanks, man) warned us that flist = [] for i in range(3) f = lambda x: x + i flist.append(f) [f(1) for f in flist] gives [3, 3, 3]. So much for the principle of minimum surprise! Doing the same in Lisp (with lists instead of arrays), (setf flist (loop for i from 0 to 2 collect (lambda (x) (+ x i)))) (loop for f in flist collect (funcall f 1)) I got (4 4 4). Lisp has many gotchas, I just wasn't ready for this one. (Google for "lisp gotchas" - someone posted a comprehensive list to c.l.l. in 1995. Every Lisper should read it) I'm sure Haskell does this right. What about Scheme and ML? Jul 18 '05 #1
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 P: n/a wrote: +--------------- | Doing the same in Lisp (with lists instead of arrays), | | (setf flist (loop for i from 0 to 2 | collect (lambda (x) (+ x i)))) | | (loop for f in flist | collect (funcall f 1)) | | I got (4 4 4). | | Lisp has many gotchas, I just wasn't ready for this one. +--------------- Why should this be considered a "gotcha"? It's doing exactly what you asked it to: all three lambdas are closed over the *same* variable binding, which was left holding "3" when the loop finished. Try it this way instead and you might get what you wanted/expected: (defparameter flist (loop for i from 0 to 2 collect (let ((u i)) (lambda (x) (+ x u))))) FLIST (loop for f in flist collect (funcall f 1)) (1 2 3) In this case the lambdas are closed over *distinct* bindings. -Rob ----- Rob Warnock 627 26th Avenue San Mateo, CA 94403 (650)572-2607 Jul 18 '05 #2

 P: n/a On Sun, 26 Oct 2003 02:53:58 -0600, rp**@rpw3.org (Rob Warnock) wrote: wrote: +--------------- | Lisp has many gotchas, I just wasn't ready for this one. +--------------- Why should this be considered a "gotcha"? Because he's a troll. Jul 18 '05 #3

 P: n/a > (setf flist (loop for i from 0 to 2 collect (lambda (x) (+ x i)))) (loop for f in flist collect (funcall f 1)) I got (4 4 4). Yes, that is suprising, although it makes more sense once you realize that they all bind to the same i, which is mutated during the loop. I'm sure Haskell does this right. What about Scheme and ML? The equivalent in Scheme (named let) introduces a new binding with each iteration, so it does what you expect. (define flist (let loop ((i 0) (r '())) (cond ((> i 2) (reverse r)) (else (loop (+ 1 i) (cons (lambda (x) (+ x i)) r)))))) (let loop ((l flist) (r '())) (cond ((null? l) (reverse r)) (else (loop (cdr l) (cons ((car l) 1) r))))) Unlike the Lisp version of flist, the Scheme loop binds a new i for each iteration. Therefore, each closure has its own i. My Scheme version is much wordier than the Lisp version above. Perhaps the more experienced schemers can show you a less verbose version that still does what you want. I've always been fond of functional languages, but I've only recently had the chance to work with them extensively, so I'm still learning. -- Bradd W. Szonye http://www.szonye.com/bradd My Usenet e-mail address is temporarily disabled. Please visit my website to obtain an alternate address. Jul 18 '05 #4

 P: n/a On Sun, 26 Oct 2003 00:11:05 -0700, mike420 wrote: [...] I'm sure Haskell does this right. What about Scheme and ML? Indeed Haskell does this right. OCaml does this right. SML doesn't have a for loop. If you emulate it with recursion idiomatic to SML (passing the incremented argument, not using a mutable reference) then it will work. Scheme doesn't have a for loop either, I think it's like in SML - or would it be more idiomatic to use "set!"? in which case it would not work. Ruby does this wrong if you use "for i in 0..2 do ... end" but right if you use "(0..2).each do |i| ... end". Smalltalk does this right, unless you use some ancient implementations which make block parameters local to the method in which they are written. I'm not sure how widespread are such implementations. Perl does this right if you remember to use "foreach my \$i (...)" instead of "foreach \$i (...)" or "foreach (...)". In the latter cases a global variable is used which is obviously wrong. I think Perl courses should emphasize "my" more. In Java I think you can't reference a mutable variable from a local class but you can reference a final variable, so it detects the problem and requires manual creation of an immutable binding to work around it. I suspect that the newer C# which will have anonymous functions does this wrong. What about Dylan? Erlang? Mercury? Moral 1: first class functions are better used with functional style (immutable data). It's because they make the time when something is evaluated harder to see, which is fine as long as data is immutable. In this example it's easy to see that the lambda is evaluated later but it's not as easy to notice that it matters that the dereferencing of the variable happens when the function is called, not when it's created. By taking away the possibility of mutation you take away some surprises. Moral 2: if you design a language with closures, it's better not to use a shared mutable variable in a "for" loop. -- __("< Marcin Kowalczyk \__/ qr****@knm.org.pl ^^ http://qrnik.knm.org.pl/~qrczak/ Jul 18 '05 #5

 P: n/a Marcin 'Qrczak' Kowalczyk wrote: Scheme doesn't have a for loop either, I think it's like in SML - or would it be more idiomatic to use "set!"? in which case it would not work. You forget do. (And for-each and map) -- Jens Axel S?gaard Jul 18 '05 #6

 P: n/a On Sun, 26 Oct 2003 11:08:12 +0100, Marcin 'Qrczak' Kowalczyk wrote: On Sun, 26 Oct 2003 00:11:05 -0700, mike420 wrote: [...] I'm sure Haskell does this right. What about Scheme and ML? Indeed Haskell does this right. OCaml does this right. Just for the record: Common Lisp also does it right. The fact that it doesn't do what someone "expects" who hasn't read the spec doesn't make its behaviour wrong. As others have pointed out you can choose if you want all the closures to capture the same binding or if you want each closure to capture a new binding. This is a feature, not a bug. Edi. Jul 18 '05 #7

 P: n/a mi*****@ziplip.com wrote: Earlier Ed Schofield (thanks, man) warned us that flist = [] for i in range(3) f = lambda x: x + i flist.append(f) [f(1) for f in flist] gives [3, 3, 3]. So much for the principle of minimum surprise! Doing the same in Lisp (with lists instead of arrays), (setf flist (loop for i from 0 to 2 collect (lambda (x) (+ x i)))) (loop for f in flist collect (funcall f 1)) I got (4 4 4). Lisp has many gotchas, I just wasn't ready for this one. (Google for "lisp gotchas" - someone posted a comprehensive list to c.l.l. in 1995. Every Lisper should read it) I'm sure Haskell does this right. What about Scheme and ML? Common Lisp does it right. (mapcar (lambda (f) (funcall f 1)) (mapcar (lambda (i) (lambda (x) (+ x i))) (list 1 2 3))) .... This is what the Haskell code eventually boild down to. It is Python that apparently cannot do this. But, in all fairness, nowhere in Python there is a claim that lambda expressions are full fledged. The LOOP based version of Common Lisp does not do what you think it does because the LOOP semantics is not the one you think it is. Of course, I can always come up with a nice set of macros that would hide some of the syntactic messiness in CL (of course do not ask me to change the evaluation rules for CL: CL is simply not lazy) Cheers -- Marco Jul 18 '05 #8

 P: n/a In comp.lang.lisp Marco Antoniotti wrote: Common Lisp does it right. (mapcar (lambda (f) (funcall f 1)) (mapcar (lambda (i) (lambda (x) (+ x i))) (list 1 2 3))) ... This is what the Haskell code eventually boild down to. It is Python that apparently cannot do this. But, in all fairness, nowhere in Python there is a claim that lambda expressions are full fledged. Python lambda isn't *that* limited. It's just that the equivalent is rather ugly by Python standards: [f(1) for f in [(lambda i: lambda x: x + i)(y) for y in [1, 2, 3]]] This also works, but isn't any prettier: map(lambda f: apply(f, (1,)), map(lambda i: lambda x: (x + i), [1, 2, 3])) (Bleah. All those colons and commas are giving me MPI flashbacks.) -- Karl A. Krueger Woods Hole Oceanographic Institution Email address is spamtrapped. s/example/whoi/ "Outlook not so good." -- Magic 8-Ball Software Reviews Jul 18 '05 #9

 P: n/a | (loop for f in flist | collect (funcall f 1)) | | I got (4 4 4). RW> Why should this be considered a "gotcha"? Because it is? The loop/collect idiom has a mostly functional feel to it, while it is implemented imperatively. It's an issue that becomes transparent once you become used to Common Lisp, just like the semantic difference between DO in CL and in Scheme. But it's still an issue. Followups restricted. Juliusz Jul 18 '05 #10

 P: n/a Q> Scheme doesn't have a for loop either, It's called DO and it does the right thing. Juliusz Jul 18 '05 #11

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