[Haskell-cafe] some questions about Template Haskell

TP paratribulations at free.fr
Mon Jul 1 23:42:16 CEST 2013

John Lato wrote:

> The problem isn't the output of nameBase, it's the input parameter 'n'. 
> In your example, you've created a function that takes input (a Name) and
> generates code based upon that input.  In order to lift a value (n) from
> an ordinary context into a quote, it needs a Lift instance.

Thanks John.
Ok I can understand that a Lift instance is needed, but to use the lift 
function below, we also need a Lift instance for the return of (nameBase n), 
because lift is a function that operates on instances of the Lift typeclass:

> :i lift
class Lift t where
  lift :: t -> Q Exp

And it is indeed the case:
> :i Lift
instance Lift a => Lift [a]
instance Lift Char

And as I have shown on a small example, lift and [||] return about the same 

> runQ $ lift "u"
ListE [LitE (CharL 'u')
> runQ $ [| "u" |]
LitE (StringL "u")

So what is the difference between lift and [||]?
Although I feel stupid, I cannot lie and claim I have understood.

> Perhaps it helps if you think about what a quote does: it allows you to
> write essentially a string of Haskell code that is converted into an AST.
>  For this to work, the quote parser needs to know how to generate the AST
> for an identifier.  Like much of Haskell, it's type-driven.  For
> identifiers in scope from imports, TH simply generates a variable with the
> correct name.  But for data, the parser needs a way to generate an AST
> representation, which is what Lift is for.

Ok, I think I understand that (we need some method to transform a value at 
data level in a token of an AST), but it seems to me it does not answer my 
question above. But I am probably wrong.



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