On Fri, Aug 7, 2020 at 9:12 PM Austin Zhu
Hello!
I'm learning Haskell and I found an interesting implementation of init using foldr. However I have difficulty understand how it works.
*init' xs = foldr f (const []) xs id* * where f x g h = h $ g (x:)*
Consider I have a input of *[1,2,3]*, then is would become
*f 1 (f 2 ( f 3 (const []))) id*
I substitute those parameters into f and the innermost one becomes *h $ (const []) (1:)*, which is simply *h []*. However when I want to reduce the expression further, I found it's hard to grasp. The next one becomes *f 2 (h [])* , which is
*h $ (h []) (2:)*
The last line isn’t correct because of erroneous alpha capture. Modulo certain things that aren’t relevant here, the definition of the folding function f is equivalent to the eta-expansion: f x g = \h -> h (g (x:)). Note the lambda abstraction. Try substituting that in *f 1 (f 2 ( f 3 (const []))) id* to see what you get. Hint: Note how f 3 (const []) evaluates to \h -> h (const [] (3:)) = \h -> h [] = ($ []) Next f 2 ($ []) becomes \h -> h (($ []) (2:)) = \h -> h (2:[]) = ($ (2:[])) And you can see how you end up with init’ [1,2,3] = 1:(2:[]) = [1,2]. Notice how I converted from a lambda abstraction to combinator form to prevent the named lambda variable h from obscuring what’s really going on. Another way to figure out this out is by calculating the precise type of the folding function f that is provided to foldr and hence the type to h. if it works like that. This looks confusing to me. To match the type of
*foldr*, h should be of type *[a] -> [a]* and *h []* would just be of type *[a]*, which isn't applicable to *(2:)*.
I also thought it in another way that *f x g* returns a function of type *([a] -> [a]) -> [a],* this kinda makes sense considering applying *id* afterwards. But then I realized I still don't know what this *h* is doing here. It looks like *h* conveys *g (x:)* from last time into the next application. Did I miss something when I think about doing fold with function as accumulator?
I'd really appreciate if anyone could help me with this. _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
-- -- Kim-Ee