[Template-haskell] change in [d| |] and creating instances in template-haskell 2.7

[Simon Peyton-Jones]

[Redireting to ghc users; the TH list is pretty dormant and I keep thinking I should close it down altogether.]

Jeremy

Actually this is by design.  See the long thread at http://hackage.haskell.org/trac/ghc/ticket/5375

When you say

| inferBar typeName =
|    do s <- [d| bar _ = "sucker"
|              |]

you are asking for a *new* definition bar _ = "sucker".  But in an instance declaration you have to mention the *existing* method name.  

To put it another way, do you expect this to work?

  do { bar_nm <- newName "bar"
     ; return (InstanceD [] <type> [FunD bar_nm <rhs>]) }

where you make up a *fresh name* (whose string-name is "bar") and use it in an instance declaration binding.

I suppose you could argue that for the odd case of instance decls, TH should ignore the exact identity of the method name, and just use its string name. It would be convenient; but another weirdness too.

User advice welcome!

Simon


| -----Original Message-----
| From: template-haskell-bounces at haskell.org [mailto:template-haskell-
| bounces at haskell.org] On Behalf Of Jeremy Shaw
| Sent: 07 September 2011 20:50
| To: template-haskell at haskell.org
| Subject: [Template-haskell] change in [d| |] and creating instances in template-
| haskell 2.7
| 
| Hello,
| 
| I have some code that likes like this, which works in template-haskell
| 2.5 / GHC 7.0.3:
| 
| ---------------
| {-# Language TemplateHaskell, TypeFamilies #-}
| module Show where
| 
| import Language.Haskell.TH
| 
| class Bar a where
|    bar :: a -> String
| 
| inferBar :: Name -> Q [Dec]
| inferBar typeName =
|    do s <- [d| bar _ = "sucker"
|              |]
|       d <- instanceD (return []) (appT (conT ''Bar) (conT typeName))
| (map return s)
|       return [d]
| 
| -----------------
| 
| $(inferBar ''Bool)
| 
| But, in template-haskell 2.6 / GHC 7.2.1, I get an error,
| 
|     Warning: No explicit method nor default method for `bar'
|      In the instance declaration for `Bar Bool'
| 
| Comparing the output of -ddump-splices we see in GHC 7.0.3/ TH 2.5, we
| have:
| 
| bar-test.hs:1:1: Splicing declarations
|      inferBar 'Bool
|    ======>
|      bar-test.hs:4:3-17
|      instance Bar Bool where
|          { bar _ = "sucker" }
| 
| But in GHC 7.2.1 / TH 2.6 we have:
| 
| bar-test.hs:1:1: Splicing declarations
|      inferBar 'Bool
|    ======>
|      bar-test.hs:4:3-17
|      instance Bar Bool where
|          { bar_acAU _ = "sucker" }
| 
| The difference being that instead 'bar' we have 'bar_acAU'.  So maybe
| that is why it can't find the method 'bar' in the instance
| declaration? Though, I would kind of expect an error like,
| 
| `bar_acAU' is not a (visible) method of class `Bar'.
| 
| Am I doing something wrong? Should I file a bug ?
| 
| Thanks!
| 
| - jeremy
| 
| 
| 
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