Re: [Haskell-cafe] Finally tagless - stuck with implementation of "lam"

BTW, here is a more symmetric version of the same code: lam f = I . return $ unsafePerformIO . unI . f . I . return which has a very clear pattern of lam f = embed $ extract . f. embed where 'embed' is often called "return". Implementations of lam in "tagless final" style tend to follow the above pattern of embed/extract, each specialized to the particular repr implementation. In some cases, one only needs a "context-sensitive" extract, i.e. an extract which is only valid in the context of an outer 'embed'. The most important example is code-generation in metaocaml where the 'compiler' version of lam reads let lam f = . .~(f .<x>.)>. where the .~ does an extract -- but *only* in the context of a surrounding code bracket (i.e. .< >. ). If there is such a context-sensitive "extract" for the IO monad, i.e. something which allows you to pretend you've got a result in IO only valid *inside* IO, then you can use that to write a nicer lam. I tried all the obvious things with monadic operators to get this done, but did not succeed in the time I had. Maybe Oleg or Ken can. So don't give up hope quite yet! Jacques Günther Schmidt wrote:

Hello Jacques,

thanks, that is disappointing in some way, guess I still have a lot to learn.

Günther

Am 05.10.2009, 18:06 Uhr, schrieb Jacques Carette :

...

It's possible, but it's not nice. You need to be able to "get out of the monad" to make the types match, i.e. lam f = I (return $ \x -> let y = I (return x) in unsafePerformIO $ unI (f y))

The use of IO 'forces' lam to transform its effectful input into an even more effectful result. Actually, same goes for any monad used inside a 'repr'.

let_ and fix follow a similar pattern (although you can hide that somewhat by re-using lam if you wish).

Jacques