Typed splices and type checking

[Simon Peyton Jones]

Good question!  See https://ghc.haskell.org/trac/ghc/ticket/10271.

Simon

| -----Original Message-----
| From: Glasgow-haskell-users [mailto:glasgow-haskell-users-
| bounces at haskell.org] On Behalf Of J. Garrett Morris
| Sent: 27 March 2015 14:30
| To: GHC users
| Subject: Typed splices and type checking
| 
| Hello,
| 
| I've run into another misunderstanding with Template Haskell and typed
| splices.  For example, I was hoping to use typed splices to generate
| earlier errors from Printf.  Here's the world's least handy printf:
| 
| > class Printf t
| >     where printf :: String -> Q (TExp String) -> Q (TExp t)
| >
| > instance Printf String
| >     where printf s t | '%' `notElem` s = [|| $$t ++ s ||]
| >                      | otherwise       = fail ("Unexpected format %"
| >                                                ++ [c])
| >               where (_, _:c:_) = break ('%' ==) s
| >
| > instance Printf t => Printf (Char -> t)
| >     where printf s t
| >             | c /= 'c' = fail ("Unexpected format %" ++ [c] ++
| >                                " for character")
| >             | otherwise = [|| \c -> $$(printf s''
| >                                          [|| $$t ++ s' ++ [c] ||])
| >                            ||]
| >               where (s', '%':c:s'') = break ('%' ==) s
| 
| Now, consider these two definitions:
| 
| > f :: Char -> String
| > f = $$(printf "foo %c" [||""||])
| 
| > h :: Char -> String
| > h y = $$(printf "foo %c" [||""||]) y
| 
| I would have expected these to be equivalent, from a type checking
| perspective.  However, while the first types fine, the second generates
| the error message:
| 
| > Printing.hs:14:12:
| >    No instance for (Printf t0) arising from a use of `printf'
| >    The type variable `t0' is ambiguous
| >    Note: there are several potential instances:
| >      instance Printf t => Printf (Char -> t)
| >        -- Defined at Printf.hs:20:10
| >      instance Printf String -- Defined at Printf.hs:9:10
| >    In the expression: printf "foo %c" [|| "" ||]
| >    In the Template Haskell splice
| >      $$(printf "foo %c" [|| "" ||])
| >    In the expression: $$(printf "foo %c" [|| "" ||])
| 
| Should I have anticipated this?  Ought the interaction of typed splices
| and eta-expansion be problematic?
| 
|  /g
| 
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