Haskell: why the convention to name a helper function “go”?

Question

I see go a lot when reading Haskell material or source, but I\'ve never been really comfortable about it - (I guess it has the negative connotation of \"goto\"

Answer 1

Hmm! Some archaeology!

Since around 2004 I've used go as the generic name for tail-recursive worker loops, when doing a worker/wrapper transformation of a recursive function. I started using it widely in bytestring, e.g.

foldr :: (Word8 -> a -> a) -> a -> ByteString -> a
foldr k v (PS x s l) = inlinePerformIO $ withForeignPtr x $ \ptr ->
        go v (ptr `plusPtr` (s+l-1)) (ptr `plusPtr` (s-1))
    where
        STRICT3(go)
        go z p q | p == q    = return z
                 | otherwise = do c  <- peek p
                                  go (c `k` z) (p `plusPtr` (-1)) q -- tail recursive
{-# INLINE foldr #-}

was from bytestring in August 2005.

This got written up in RWH, and probably was popularized from there. Also, in the stream fusion library, Duncan Coutts and I started doing it a lot.

From the GHC sources

The idiom goes back further though. foldr in GHC.Base is given as:

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

which is probably where I picked up the trick (I'd thought this was from Andy Gill's thesis, but can't find any use of go there). It isn't given in this form in Gofer, so I think this first appeared in the GHC code base.

By 2001, Simon Marlow was using go in some of the systems-level code, so we might place the blame somewhere in GHC, and this clue leads us to the GHC source, where go is widely used in worker functions:

myCollectBinders expr
  = go [] expr
  where
    go bs (Lam b e)          = go (b:bs) e
    go bs e@(Note (SCC _) _) = (reverse bs, e)
    go bs (Cast e _)         = go bs e
    go bs (Note _ e)         = go bs e
    go bs e                  = (reverse bs, e)

GHC 3.02 and Glasgow

Digging up old versions of GHC, we see that in GHC 0.29 this idiom does not appear, but by GHC 3.02 series (1998), the go idiom appears everywhere. An example, in Numeric.lhs, in the definition of showInt, dated to 1996-1997:

showInt n r
  | n < 0     = error "Numeric.showInt: can't show negative numbers"
  | otherwise = go n r
    where
     go n r =
      case quotRem n 10 of                 { (n', d) ->
      case chr (ord_0 + fromIntegral d) of { C# c# -> -- stricter than necessary
      let
    r' = C# c# : r
      in
      if n' == 0 then r' else go n' r'
      }}

this is a different implementation to the one given in the H98 report. Digging into the implementation of "Numeric.lhs", however, we find that it isn't the same as the version that was added to GHC 2.06 in 1997, and a very interesting patch from Sigbjorne Finne appears, in April 1998, adding a go loop to Numeric.lhs.

This says that at least by 1998, Sigbjorne was adding go loops to the GHC "std" library, while simultaneously, many modules in the GHC compiler core had go loops. Digging further, this very interesting commit from Will Partain in July 1996 adds a "go" loop into GHC -- the code comes from Simon PJ though!

So I'm going to call this as a Glasgow idiom invented by people at Glasgow who worked on GHC in the mid 90s, such as Simon Marlow, Sigbjorn Finne, Will Partain and Simon Peyton Jones.

Answer 2

Obviously Don's answer is the correct one. Let me just add in a small detail (since it seems to be my writing that you're directly referring to): go is nice because it's only two letters.

Oh, and the reason the Yesod book devotes so much content to the enumerator package is because I'd already written up the three-part tutorial of enumerator as a blog post series, so decided I may as well include it in the book. The enumerator package is used in a number of places throughout Yesod, so it is relevant.

Answer 3

I'd expect this idiom to be applicable not just to linear structures (and hence "loops"), but also to branching (tree-like) structures.

I wonder how often the go pattern corresponds to accumulation parameters and, more generally, with the continuation-encoding strategites that Mitch Wand explored in the paper Continuation-Based Program Transformation Strategies (one of my all-time favorite papers). In these cases, the go function has a particular meaning, which can then be used to derive efficient code from an elegant specification.