On Sep 8, 2011, at 4:00 AM, Simon Peyton-Jones wrote:
[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.
Right. That makes sense.
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.
no.
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.
Yeah. I would expect this to work: inferBar2 :: Name -> Q [Dec] inferBar2 typeName = [d| instance Bar $(conT typeName) where bar _ = "sucker" |] But I get the same error: inferBar2 'Bool ======> show-test.hs:4:3-18 instance Bar Bool where { bar_aTK _ = "sucker" } show-test.hs:4:3: Warning: No explicit method nor default method for `bar' In the instance declaration for `Bar Bool' Presumably because bar is still being created as a *fresh name*. I think in that version, it is more surprising that it does not work because the whole instance declaration is inside the [d| |]. Additionally, it is not obvious (to me) how to work around the issue and keep the code pretty / easily readable. But, as you point out, making bar not be a fresh name there creates a 'special case'. So, that is not great either.. When you saw inferBar2, did you find it somewhat 'surprising' that it didn't work ? - jeremy
User advice welcome!
Simon
| -----Original Message----- | From: template-haskell-bounces@haskell.org [mailto:template-haskell- | bounces@haskell.org] On Behalf Of Jeremy Shaw | Sent: 07 September 2011 20:50 | To: template-haskell@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 | | | | _______________________________________________ | template-haskell mailing list | template-haskell@haskell.org | http://www.haskell.org/mailman/listinfo/template-haskell