Constructing TH types
Eric M. Pashman
eric.pashman at gmail.com
Sun Oct 7 19:14:53 CEST 2012
Whoops, sorry for the bad code in my last message. I mean of course that the problem is that
ConT (mkName "") /= ListT
On Oct 7, 2012, at 12:08 , Eric M. Pashman <eric.pashman at gmail.com> wrote:
> Intended usage seems to be something like
> AppT ListT $ ConT . mkName $ "Int"
> That works. So it seems that there is indeed no way to construct the syntactically special list type from its (valid) name, but it can easily be done as above.
> In my opinion, it should be possible to do what we've been trying -- ConT (mkName "") `AppT` ConT "Char" -- or whatever. The problem seems simply to be that
> ConT . mkName $ "" \= ListT
> I don't know the first thing about the GHC source, but I would imagine that's easily fixable if the behavior we both expected is more broadly desired.
> On Oct 7, 2012, at 11:24 , Nicolas Frisby <nicolas.frisby at gmail.com> wrote:
>> The issue you had with applications of the  type seems to be more
>> insidious than my last email made it out to be. This expression
>> ( $(return $ ConE (mkName "")) ::
>> $(return $ ConT (mkName "") `AppT` ConT ''Char)
>> fails with " is applied to too many arguments". I'm thinking that
>> the "" in the type is being resolved somehow to the the  data
>> constructor, not the  type constructor.
>> This seems specific to ; the following works for tuples, even though
>> the data and type constructor also share a string name.
>> $(return $ ConE (mkName "(,)") `AppE` LitE (CharL 'c') `AppE` LitE
>> (CharL 'a')) ::
>> $(return $ ConT (mkName "(,)") `AppT` ConT ''Char `AppT` ConT ''Char)
>> I haven't yet narrowed down where the mkName'd string is
>> (inappropriately?) resolved in the GHC source.
>> Eric, special-casing for ArrowT probably avoids this problem for you.
>> The only thing to glean from this email is that you ideally wouldn't
>> need to worry about the special-casing for your current application —
>> I think there's a TH bug at play, though I haven't found an open GHC
>> ticket for it and it may very well still be a known issue.
>> On Sun, Oct 7, 2012 at 10:15 AM, Eric M. Pashman <eric.pashman at gmail.com> wrote:
>>> Nicolas, thanks for the welcome, and thanks for pointing out the additional 'Type' constructors!
>>> I'd looked over the available constructors, but apparently not very well. The 'ListT', 'TupleT', 'ArrowT', etc., constructors are precisely what I need to make this work in a straightforward fashion.
>>> So I don't actually have a problem making the right 'Name' value, just a problem reading the TH source. Well, that's that. ...
>>> Many thanks,
>> My pleasure.
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