[Haskell] "strange" behavior of Implicit Parameters

[Eike Scholz]

Hi,

I have just a short question, about the semantics of Implicit parameters
on GHC 6.4.

Given the following code: 

> (???) x f 
>  = let ?foo = 1337
>     in f x

> fun :: (?foo::Int,Show x) => x -> String
> fun x =  "x = "++(show x)++"; ?foo = "++(show ?foo) 

> test = let ?foo = 23
>         in 42 ??? fun

I would expect:

test == "x = 42; ?foo = 1337"

Because I thought, that it gets reduced like:

let ?foo = 23   in 42 ??? fun
~> let ?foo = 23 in (let ?foo = 1337 in fun 42)
~> ...
~> "x = 42; ?foo = 1337"

however I get

"x = 42; ?foo = 23"

in ghci.

Is there a special reason, why the implicit parameter is reduced first?
Could it be a bug?

Reducing it my way would be very useful for combinator-libraries, since
it could be used passing around state info, or simple debug text, that
tells where a Runtime-error occured?

an odd example:


> undef = error ?errstr

> (<?>) x s = let ?errstr = ?errstr++"\n"++s in x -- annotate error tags


so error annotations will accumulate and it would be easy to resolve
runtime errors.  


best regards

   Eike Scholz