[Haskell-cafe] Documentation operator

[Christopher Done]

Hello chums,

I've been playing around with an idea, something that has obvious pros
and cons, but I'll sell it to you because there might be some positive
ideas out of it. Consider the following operator:

    {-# LANGUAGE TypeOperators, DataKinds, KindSignatures #-}

    module Docs where

    import GHC.TypeLits

    type a ? (sym :: Symbol) = a

First I'll describe how I'd want to use this and then what I think
are the advantages and disadvantages.

I call this (?) operator “the documentation operator”, to be used for:

* Things that either don't belong or can't be encoded in the type
  system, or for things need to be in English.
* Things that cannot be encoded in Haddock.

The simple case of ye olde days:

    -- | Lorem ipsum dolor sit amet. Suspendisse lacinia nibh et
    --   leo. Aenean auctor aliquam dapibus.
    loremIpsum :: Int -> Int -> String

Which has since been somewhat evolved into:

    loremIpsum :: Int    -- ^ Lorem ipsum dolor sit amet.
               -> Int    -- ^ Suspendisse lacinia nibh et leo.
               -> String -- ^ Aenean auctor aliquam dapibus.

But could now be written:

    loremIpsum :: Int    ? "Lorem ipsum dolor sit amet."
               -> Int    ? "Suspendisse lacinia nibh et leo."
               -> String ? "Aenean auctor aliquam dapibus."

Here is a contrived case I'll use later on:

    data Person = Person

    describeAge :: Int ? "an age" -> String ? "description of their
elderliness"
    describeAge n = undefined

    personAge :: Person ? "a person" -> Int ? "their age"
    personAge = undefined

One could also encode previously informal specifications more formally,
so that

    -- | The action 'hFlush' @hdl@ causes any items buffered for output
    -- in handle @hdl@ to be sent immediately to the operating system.
    --
    -- This operation may fail with:
    --
    --  * 'isFullError' if the device is full;
    --
    --  * 'isPermissionError' if a system resource limit would be exceeded.
    --    It is unspecified whether the characters in the buffer are
discarded
    --    or retained under these circumstances.
    hFlush :: Handle -> IO ()
    hFlush handle = wantWritableHandle "hFlush" handle flushWriteBuffer

with

type Throws ex (docs :: Symbol) = docs

could now be written

    hFlush :: Handle ? "flush buffered items for output on this handle" ->
IO ()
      ? Throws IsFullError "if the device is full"
      ? Throws IsPermissionError
               "if a system resource limit would be exceeded. It is \
               \unspecified whether the characters in the  buffer are \
               \discarded or retained under these circumstances."
    hFlush handle = wantWritableHandle "hFlush" handle flushWriteBuffer

With this in place, in GHCi you get documentation "lookup" for free:

    > :t hFlush
    hFlush
      :: (Handle ? "flush buffered items for output on this handle")
         -> (IO () ? Throws IsFullError "if the device is full")
            ? Throws
                IsPermissionError
                "if a system resource limit would be exceeded. It is
unspecified
                 whether the characters in the  buffer are discarded or
retained
                 under these circumstances."

And you get function composition, or “documentation composition” for free:

    > :t describeAge . personAge
    describeAge . personAge
      :: (Person ? "a person")
         -> String ? "description of their elderliness"

We could have a :td command to print it with docs, and otherwise docs
could be stripped out trivially by removing the ? annotations:

    > :t describeAge . personAge
    describeAge . personAge
      :: Person -> String
    > :td describeAge . personAge
    describeAge . personAge
      :: (Person ? "a person")
         -> String ? "description of their elderliness"

You could even add clever printing of such “documentation types”:

    > :t hFlush
    hFlush
      :: Handle — flush buffered items for output on this handle
      -> IO ()
    Throws IsFullError if the device is full"
    Throws IsPermissionError if a system resource limit would be
      exceeded. It is unspecified whether the characters in the buffer
      are discarded or retained under these circumstances."

Unfortunately it doesn't work with monadic composition, of course.

So here are the advantages:

* You get parsing for free (and anyone using haskell-src-exts).
* You get checking for free (i.e. GHC can check that IsFullError exists
  for you).
* You get a continuity of documentation through your operations
  including composition.
* You can extend the "documentation language" easily by just defining
  some types (like the Throws I used above). SeeMore, Author,
  Deprecated, etc. Whatever.
* You can print out some helpful looking documentation in GHCi based on
  these simple types.
* There's no longer this informal "it might throw this exception" kind
  of pros we're forced to write.
* It could also be used for annotations other than pure documentation,
  including testing. E.g. add a Testable "property" and then your test
  framework can search for functions with this Testable annotation.
* Free of Haddock's syntax.

Here are the disadvantages:

* It doesn't work for types.
* Writing big pros inside a string can be boring without a decent
  editor.
* The neat composition trick only goes so far.
* There might be a compilation overhead.
* It would require an updated GHCi to strip them out when not wanted.
* Requires GHC 7.6.1+.

Conclusions:

What we have now for documentation is pretty good, especially generated
documentation. Compared to other languages Haskell is quite well
documented, I feel. But we can do more with it. In some languages,
documentation is built into the language. You can ask for documentation
inside the REPL, it belongs to that piece of code. It shouldn't, I don't
think, be left as a code comment which is essentially whitespace as far
as the compiler is concerned.

Two sweet ideas that I like from the above are:

* The checking by GHC.
* The extension of the "documentation language", with the ability to
  formalize things like what exceptions are thrown.
* Composing functions generates "new" documentation that still makes
  sense.

Thoughts?

Ciao!
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