Typed splices and type checking

[J. Garrett Morris]

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

-- 
The University of Edinburgh is a charitable body, registered in
Scotland, with registration number SC005336.