Proposal: ValidateMonoLiterals - Initial bikeshed discussion

Dominique Devriese dominique.devriese at
Fri Feb 6 12:13:33 UTC 2015


Perhaps only for the sake of discussion: have you considered doing
something at the type-level instead of using TH? I mean that you could
change the type of 42 from `forall a. Num a => a` to `forall a.
HasIntLiteral a '42 => a` where HasIntegerLiteral is a type class of
kind `* -> 'Integer -> Constraint` and people can instantiate it for
their types:

class HasIntegerLiteral (a :: *) (k :: 'Integer) where
  literal :: a

The desugarer could then just generate an invocation of "literal".

An advantage would be that you don't need TH (although you do need
DataKinds and type-level computation).  Specifically, type-checking
remains decidable and you can do it in safe haskell and so on.  I
haven't thought this through very far, so there may be other
advantages/disadvantages/glaring-holes-in-the-idea that I'm missing.


2015-02-06 11:07 GMT+01:00 Merijn Verstraaten <merijn at>:
> And no one of my proofreaders noticed that >.>
> I would propose to have the extension replace the 'fromString "foo"', 'fromIntegral 5' and 'fromList [1,2,3]' calls (for monomorphic cases) in the AST with the relevant Typed TH splice.
> I considered quasi-quotation initially too, but there's no quasi quotation syntax for Typed TH. I'm guessing that's just an oversight, but I'd really be in favour of adding a typed quasiquoter too. Similarly to thinking we should have an easier way to obtain Lift  instances since, to me at least, it seems that the Lift instance for most ADTs should be fairly trivial?
> I'll quickly clarify the proposal on the wiki :)
> Cheers,
> Merijn
>> On 5 Feb 2015, at 22:48, Simon Peyton Jones <simonpj at> wrote:
>> I'm all for it.  Syntax sounds like the main difficulty.  Today you could use quasiquotatoin
>>       [even| 38 |]
>> and get the same effect as $$(validate 38).  But it's still noisy.
>> So: what is the non-noisy scheme you want to propose?  You don't quite get to that in the wiki page!
>> Simon
>> | -----Original Message-----
>> | From: ghc-devs [mailto:ghc-devs-bounces at] On Behalf Of Merijn
>> | Verstraaten
>> | Sent: 05 February 2015 14:46
>> | To: ghc-devs at; GHC users
>> | Subject: Proposal: ValidateMonoLiterals - Initial bikeshed discussion
>> |
>> | I've been repeatedly running into problems with overloaded literals and
>> | partial conversion functions, so I wrote up an initial proposal
>> | ( and I'd like
>> | to commence with the bikeshedding and hearing other opinions :)
>> |
>> | Cheers,
>> | Merijn
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