Hi,
I think that this is a neat idea that should be explored more! GHC's
parser has a bunch of awkward duplication to handle attaching documentation
to types, and it'd be cool if we could replace it with an actual language
construct.
Happy holidays!
-Iavor
On Wed, Dec 26, 2012 at 3:27 AM, 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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