[Template-haskell] quasi quotes and Q monad

[Tomasz Zielonka]

On Tue, Jan 03, 2006 at 04:13:53PM +0100, Ch. A. Herrmann wrote:
> the comparison doesn't work with the IO monad because the user runs the
> Q monad every time when splicing a generated expression.

I am not sure about it. At least it seems that the "name generation
state" is passed between splices. Try this:

    module Gen where

    import Language.Haskell.TH

    gen :: Q [Dec]
    gen = do
        a <- newName "a"
        b <- newName "b"
        expr <- [| (1, 2) |]
        return [ValD (TupP [VarP a, VarP b]) (NormalB expr) []]

 ---------------------------------------- 

    module X where

    import Gen

    $(gen)
    $(gen)
    $(gen)
    $(gen)
    $(gen)

:browse X shows something like this:

    a[a2ss] :: Integer
    a[a2us] :: Integer
    a[a2vj] :: Integer
    a[a2wa] :: Integer
    a[a2x1] :: Integer
    b[a2su] :: Integer
    b[a2uu] :: Integer
    b[a2vl] :: Integer
    b[a2wc] :: Integer
    b[a2x3] :: Integer

BTW, you can use such a definition g:

    (\x -> [| h $(return x) |])

now x can be Expr. It's just moving "return" into g.

Best regards
Tomasz

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