Proposal: Partial Type Signatures

[Thomas Winant]

Dear GHC developers,

Together with Tom Schrijvers, Frank Piessens and Dominique Devriese, I
have been working on a proposal for adding *Partial Type Signatures* to
GHC. In a partial type signature, annotated types can be mixed with
inferred types. A type signature is written like before, but can now
contain wildcards, written as underscores. The types of these wildcards
or unknown types will be inferred by the type checker, e.g.

     foo :: _ -> Bool
     foo x = not x
     -- Inferred: Bool -> Boo

The proposal also includes a form of generalisation which aligns with
the existing generalisation that GHC does. We have written down a
motivation (when and how might you use this) and details about the
design and implementation on the following wiki page:

https://ghc.haskell.org/trac/ghc/wiki/PartialTypeSignatures

We have a (work in progress) implementation [1] of the feature based on
GHC. It currently implements most of what we propose, but there are some
remaining important bugs mostly concerning the generalisation. We also
described our design and presented a formalisation based on the
OutsideIn(X) formalism in a paper [2] presented at PADL'14.

What we are hoping to get from the people on this list is any of the
below:
* Read the design, play with the implementation and tell us any comments
   you may have about the feature, its design and implementation.
* Opinions on whether this feature might be acceptable in GHC upstream
   at some point (if not, we do not think it's worth developing the
   implementation much further).
* Perhaps a code review or a discussion with someone more knowledgeable
   about the internals of GHC's type checker about how we might fix the
   remaining problems in our implementation (specifically, we could use
   some help with implementing the generalisation of partial type
   signatures).
* Feedback on the `Questions and issues' section on the wiki page.


Kind regards,
Thomas Winant

[1]: https://github.com/mrBliss/ghc-head/
[2]: https://lirias.kuleuven.be/bitstream/123456789/423475/3/paper.pdf


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