o...@okmij.org wrote: >> pr :: Name -> ExpQ >> pr n = [| putStrLn $ (nameBase n) ++ " = " ++ show $(varE n) |] > > The example is indeed problematic. Let's consider a simpler one: > >> foo :: Int -> ExpQ >> foo n = [|n + 1|] > > The function f, when applied to an Int (some bit pattern in a machine > register), produces _code_. It helps to think of the code > as a text string with the > source code. That text string cannot include the binary value that is > n. That binary value has to be converted to the numeric text string, and > inserted in the code. That conversion is called `lifting' (or > quoting). The function foo is accepted because Int is a liftable type, > the instance of Lift. And Name isn't.
Thanks Oleg, Probably the following question will be stupid, but I ask it anyway: in my initial example, (nameBase n) returns a String, so we are not in the case where it is not "liftable"? In fact I am not sure to have understood your answer. Now, I have found another behavior difficult to understand for me: > runQ $ lift "u" ListE [LitE (CharL 'u') > runQ $ [| "u" |] LitE (StringL "u") So we have similar behaviors for lift and [||]. We can check it in a splice: > $( [| "u" |] ) "u" > $( lift "u" ) "u" But if I replace a working version: pr n = [| putStrLn ( $(lift( nameBase n ++ " = " )) ++ show $(varE n) ) |] by pr n = [| putStrLn ( $([| (nameBase n) ++ " = " |]) ++ show $(varE n) ) |] I again get the error """ No instance for (Lift Name) arising from a use of `n' Possible fix: add an instance declaration for (Lift Name) In the first argument of `nameBase', namely `n' """ It is not easy to surmise the reason for this error. TP _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
