Proposal: liftData for Template Haskell

[Michael Sloan]

+1 to liftData

Removing 'Lift' altogether definitely isn't the way to go, though. As
Richard points out, we want to be able to lift more than just ADTs.  Also
ADTs which hide their implementation can have either opaque Data instances,
no Data instance, or Data instances which involve using functions for
constructors.

An example of the latter is Data.Map.Map's Data instance, which uses
'fromList' as a constructor.  This results in $(dataToExpQ (\_ -> Nothing)
(fromList [(1,2)])) causing the compiletime error "Illegal data constructor
name: ‘fromList’".

I think 'dataToExpQ' and related functions should be modified to handle
this case.  It should be rather easy - if the constructor Name is
lowercase, generate a 'VarE' instead of a 'ConE'.  I suppose this is a
separate proposal, but it came up when thinking about this proposal.

-Michael



On Fri, Apr 17, 2015 at 4:21 AM, Edward Z. Yang <ezyang at mit.edu> wrote:

> I propose adding the following function to Language.Haskell.TH:
>
>     -- | 'liftData' is a variant of 'lift' in the 'Lift' type class which
>     -- works for any type with a 'Data' instance.
>     liftData :: Data a => a -> Q Exp
>     liftData = dataToExpQ (const Nothing)
>
> I don't really know which submodule this should come from;
> since it uses 'dataToExpQ', you might put it in Language.Haskell.TH.Quote
> but arguably 'dataToExpQ' doesn't belong in this module either,
> and it only lives there because it is a useful function for defining
> quasiquoters and it was described in the quasiquoting paper.
>
> I might propose getting rid of the 'Lift' class entirely, but you
> might prefer that class since it doesn't go through SYB (and have
> the attendant slowdown).
>
> This mode of use of 'dataToExpQ' deserves more attention.
>
> Discussion period: 1 month
>
> Cheers,
> Edward
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