example from page 59 of TAPL not working quite as expected? about mal

[kanaka]

I think you have a couple of typos. You test function is missing some parens:

(def! test (fn* [l m n] (l m n)))

And your fls function has an incorrect variable name:

(def! fls (fn* [t f] f))

Making those changes I get the correct behavior.

[igravious]

Ninja edited my typo up above, that wasn't the issue. It seems () around multiple variables gives me the correct behaviour. Is that Lispful that I need () around multiple variables? For what it's worth Haskell doesn't need them though that may be no indication. let test = (\l -> \m -> \n -> l m n)

[kanaka]

Parens in Lisp define lists. However, when an unquoted list is evaluated in Lisp, the first position in the list is a function and the rest of the elements are arguments. So (+ 2 3) means call the + function using the arguments 2 and 3. In mal, (fn* (a b) (+ a b)) means define an anonymous function that takes two arguments and calls + using those arguments. This (fn* (a b) + a b) is quite different. That's an anonymous function containing three elements that are not in a list. Mal only uses the first element in the function body so it's really equivalent to this: (fn* (a b) +). When you invoke/apply that function you'll just get what + evaluates to. In other words, this ( (fn* (a b) +) 7 8) will invoke/apply the anonymous function, the + symbol will be evaluated to the native function for addition which will be returned.

It's late at night and I'm tired, so the above explanation might be a bit rambling, let me know if it doesn't make sense and I'll try and give a better answer tomorrow after some sleep.