[Caml-list] List.fold_left vs. Hashtbl.fold

[William Smith <bills@emu-bark.com>]

That seems like an easy mnemonic--left means the base case argument is on the left and right means that the base case is on the right.   This is both for the parameters to fold_left/right and the parameters of its <fun> argument.  And then, if there is not _right/_left distinction available for a container, it defaults to being equivalent to _right.

Before I saw other people's explanations, I was thinking about how the parameter order fold_left is more useful for currying.

If you want to create an operator, fold_left makes it a lot easier to curry in its base case and then apply the operator to different lists that the operator is processing last.

If I wanted to do that with fold_right or the other container's folds, I would have to write a wrapper to swap the arguments--which isn't really a big deal, but it's a way that fold_left is more useful that no one mentioned.

Bill

> On Wed, Nov 28, 2012 at 6:25 PM, Gabriel Scherer
> <gabriel.scherer@gmail.com>  wrote:

> That's for the type. Regarding the implementation, fold_left applies
> the argument function starting from "the left" (the beginning) of the
> list first, while fold_right begins "on the right" (at the end): for
> some infix operator (++) :
>   fold_left (++) init [x; y; z]     is  ((init ++ x) ++ y) ++ z
>   fold_right (++) [x; y; z] init   is (x ++ (y ++ (z ++ init)))
> (The evaluation "begins" in the innermost nested parentheses)

On Wed, Nov 28, 2012 at 5:40 AM, William Smith<bills@wwayneb.com>  wrote:

> Hashtbl.fold expects the Hasthbl as the second parameter with the 3rd
> parameter being the initial value... just the opposite of List.fold_left.