Formatting function types

A very common way (at least in the base libraries) of formatting function types seems to be this:

hPutBuf :: Handle                       -- handle to write to
        -> Ptr a                        -- address of buffer
        -> Int                          -- number of bytes of data in buffer
        -> IO ()

I remember when I first started learning Haskell, and these many-arrowed functions seemed very strange to me: "Okay, we give it a handle, and get a pointer, and, um, from this we get an int, and from this an action? Er?"

I'd like to interject here for a moment. The way to read a Signature in that style is similar to reading a multiline summation. e.g

hPutBuf :: Handle -> Ptr a -> Int -> IO ()

hPutBuf: Handle + Ptr a + Int ------------- = IO ()

I aggree with the argument, that for beginners it is hard to see what a function returns, but that has to do with currying. A right associative way for defining a function signature would (only in this case and worse in others) be easier to read.

The problem here is that the first parameter has a distinguished look while the other parameters and the return value all look the same. I think that a naive reader is inclined to assume that line breaks are situated at major structural boundaries. Consider two different interpretations of the structure of the type term:

  (((Handle               (Handle
  -> Ptr a)             ->(Ptr a
  -> Int)               ->(Int
  -> IO ())             -> IO ())))

Which one looks more natural?

The point of this rant is just this: the aforementioned multi-line formatting style should only be used for left-associative infix operators. For right-associative ones (such as the function arrow), the One True Way is this:

Handle ->
Ptr a ->
Int ->
IO ()

Unfortunately the first (misleading) style is used by Haddock for library documentation and GHC for error reporting.

See also

• Lauri Alanko in Haskell Cafe on Formatting function types
• and once again with more participants