Resolved+new Q: TypeLits question, how to build a Type Application with Symbol index

[Simon Peyton Jones]

| Yes I have a branch, and it works! A bunch of things is still missing
| (notably record selectors), but I have a proof-of-concept with a gdiff
| library hooked up to GHC.Generics, and by appealing to type-level
| reasoning I can obtain a difference tree from True to False (which
| looks good) by using the reflection (i.e. class Generic) only, no need
| for TH or hand-coding. Comparing bigger trees (and then 'patch'ing
| them) appears to be SMOP from here.

Do you have a wiki page explaining what "it" is (the thing that works).

| instance Datatype (Dat "MyModule" "Foo") ...
| 
| I get an 'orphan instance' warning. I believe that these are harmless,

The downside of orphan instances is that GHC must visit every .hi file that has an orphan instance, just in case it contains a relevant instance decl.  That slows down *every* compilation, whether or not it uses the instance.

The best way to get rid of it is to declare something local that is "from this module". Something like

  data MyModule_Foo
  instance DataType (Dat MyModule_Foo) where ...

Now MyModule_Foo is a data type from the module currently being compiled.  That tells GHC which .hi file to look in, and means the instance isn't orphan.

Simon

| so is there a way to suppress them? Since I never insert tyvars in the
| instance head, there should never be any overlap too.
| 
| Cheers,
| 
|     Gabor
| 
| On 6/30/14, Simon Peyton Jones <simonpj at microsoft.com> wrote:
| > You'll need to give a lot more info than this before I can help Gabor.
| > Currently I have only the vaguest idea about what you are trying to
| > accomplish.  Is there a wiki page that describes the design (user's eye
| > view) in detail?
| >
| > I see you have a branch.  If you are stuck, and give me repro
| instructions,
| > I can attempt to help.
| >
| > Simon
| >
| > | -----Original Message-----
| > | From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of
| Gabor
| > | Greif
| > | Sent: 27 June 2014 17:51
| > | To: ghc-devs
| > | Subject: Resolved+new Q: TypeLits question, how to build a Type
| > | Application with Symbol index
| > |
| > | I succeeded to solve all of them :-)
| > |
| > | But now I am blocked on on a panic
| > |
| > | "not in scope during type checking, but it passed the renamer".
| > |
| > | I suspect that while "deriving Generic" some instances are defined in
| > | some empty TcEnv, which does not contain my definition in context.
| > |
| > | Is there a way to inject some type constructor into the TcEnv?
| > |
| > | Thanks,
| > |
| > |     Gabor
| > |
| > | On 6/27/14, Gabor Greif <ggreif at gmail.com> wrote:
| > | > Hello devs,
| > | >
| > | > I have
| > | >
| > | > {{{
| > | > data D (n :: Symbol)
| > | > }}}
| > | >
| > | > in my module, and I want to obtain a type
| > | >
| > | > {{{
| > | > D "YAY!"
| > | > }}}
| > | >
| > | > programmatically. Where can I find code that performs this (or
| > | > something similar)?
| > | >
| > | > 1) I have to look up |D| in the current TyEnv (what if it is in a
| > | > specific module?),
| > | > 2) I have to build the type index (of kind Symbol), this involves
| > | > FastString, looks non-trivial,
| > | > 3) Apply 1) on 2), this is easy.
| > | >
| > | > Any hints welcome!
| > | >
| > | > Thanks and cheers,
| > | >
| > | >     Gabor
| > | >
| > | >
| > | > PS: some morsels I have so far:
| > | >
| > | > for 1)
| > | > compiler/prelude/PrelNames.lhs:gHC_GENERICS    = mkBaseModule
| (fsLit
| > | > "GHC.Generics")
| > | >
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| >