I'm reading the following rule from your answer: [|exp|] normally returns the unevaluated AST of exp. However, if exp contains local variables, these are lifted using Language.Haskell.TH.lift (i.e. evaluated before lifting). Is that correct? / Emil On 2008-03-13 09:49, Alfonso Acosta wrote:

Hi Emil,

Your problem is related to "how are things evaluated" not "when". The short answer is: if you want to make sure an expression is evaluated before you lift it, don't use quasiquotes, call Language.Haskell.TH.lift

On Thu, Mar 13, 2008 at 9:00 AM, Emil Axelsson wrote:

...

a1 = [| (2::Int) + 2 |]

You are lifting the expression AST, not its evaluation. a1 = lift ((2::Int) + 2) would work as you want.

...

a2 = let x = (2::Int) + 2 in [| x |]

here you are enclosing a local variable in quasiquotes and, thus, [| x |] is equivalent to "lift x"

...

a3 = [| y |] where y = (2::Int) + 2

Same as in a2, y is local. Therefore [| y |] is equivalent to "lift y"

...

z = (2::Int) + 2

a4 = [| z |]

z is a global variable and [| z |] is lifted to a variable expression (i.e. a4 is equivalent to "varE 'z" )

| > I'm reading the following rule from your answer: | > | > [|exp|] normally returns the unevaluated AST of exp. However, if exp | contains | > local variables, these are lifted using Language.Haskell.TH.lift (i.e. | evaluated | > before lifting). | > | > Is that correct? | > | > | > / Emil | | Yes, that seems to be true. I'm not an expert in the internals of TH | though, so I have inferred that rule by extensive use of TH ;). | | SPJ can confirm if it's right. Sorry, been busy with the ICFP deadline. I think you are asking this: module M(f) where f :: Int -> Q Exp f x = let expensive :: Int -> Int expensive p = p*p + x*x in let y = expensive x in [| y+1 |] module Test where import M test n = n + $(f 4) When compiling module Test, TH will evaluate (f 4), returning a syntax tree which it will splice in place of the call $(f 4). What expression will it return? Two candidates: $(f 4) --> 24+1 $(f 4) --> expensive 4 + 1 In TH you get the former, which is I think what you understood. Why? Apart from anything else, 'expensive' isn't even in scope in module Test -- it was a local binding inside the invocation of f. Second, this is partly what staging is about; you get to specify when you want things to be done. If you want the splice to contain the call to expensive (rather than its result), you'll need to float out expensive to the top level (which means lambda-lifting). And then you can say this: expensive :: Int -> Int -> Int expensive x p = p*p + x*x f :: Int -> Q Exp f x = let y = [| expensive x x |] in [| $y+1 |] By putting the call in a quote we delay its evaluation. If someone felt like transcribing this little thread into a FAQ-like thing on the GHC user wiki (I'm disconnected at the moment) that would be a fine thing to do. Thanks. Simon