haskell - Why does foldr use a helper function?

Question

In explaining foldr to Haskell newbies, the canonical definition is

foldr            :: (a -> b -> b) -> b -> [a] -> b
foldr _ z []     =  z
foldr f z (x:xs) =  f x (foldr f z xs)

But in GHC.Base, foldr is defined as

foldr k z = go
          where
            go []     = z
            go (y:ys) = y `k` go ys

It seems this definition is an optimization for speed, but I don't see why using the helper function go would make it faster. The source comments (see here) mention inlining, but I also don't see how this definition would improve inlining.

Answer 1

I can add some important details about GHC's optimization system.

The naive definition of foldr passes around a function. There's an inherent overhead in calling a function - especially when the function isn't known at compile time. It'd be really nice to able to inline the definition of the function if it's known at compile time.

There are tricks available to perform that inlining in GHC - and this is an example of them. First, foldr needs to be inlined (I'll get to why later). foldr's naive implementation is recursive, so cannot be inlined. So a worker/wrapper transformation is applied to the definition. The worker is recursive, but the wrapper is not. This allows foldr to be inlined, despite the recursion over the structure of the list.

When foldr is inlined, it creates a copy of all of its local bindings, too. It's more or less a direct textual inlining (modulo some renaming, and happening after the desugaring pass). This is where things get interesting. go is a local binding, and the optimizer gets to look inside it. It notices that it calls a function in the local scope, which it names k. GHC will often remove the k variable entirely, and will just replace it with the expression k reduces to. And then afterwards, if the function application is amenable to inlining, it can be inlined at this time - removing the overhead of calling a first-class function entirely.

Let's look at a simple, concrete example. This program will echo a line of input with all trailing 'x' characters removed:

dropR :: Char -> String -> String
dropR x r = if x == 'x' && null r then "" else x : r

main :: IO ()
main = do
    s <- getLine
    putStrLn $ foldr dropR "" s

First, the optimizer will inline foldr's definition and simplify, resulting in code that looks something like this:

main :: IO ()
main = do
    s <- getLine
    -- I'm changing the where clause to a let expression for the sake of readability
    putStrLn $ let { go [] = ""; go (x:xs) = dropR x (go xs) } in go s

And that's the thing the worker-wrapper transformation allows.. I'm going to skip the remaining steps, but it should be obvious that GHC can now inline the definition of dropR, eliminating the function call overhead. This is where the big performance win comes from.