[Template-haskell] quasi quotes and Q monad

[Ch. A. Herrmann]

Hi Tomasz,

Tomasz Zielonka wrote:

>On Mon, Jan 02, 2006 at 05:16:36PM +0100, Ch. A. Herrmann wrote:
>  
>
>>The value I want to pass for x is of type Exp. Of course, I could turn
>>this type into (Q Exp) by applying return, but this artificial instance 
>>of the Q monad would come from nowhere, not being connected with the
>>regular instance used, e.g., for the fresh name generation.
>>    
>>
>
>It seems that you are a bit confused with monads. The only monad you
>have here is the Q type constructor. 
>
maybe you misunderstood me. Take the example of the ST monad. If you are 
in an ST monadic
computation with state, then apply a non-monadic function in which you 
run a ST monad
again, you are faced with two different states. As long as you finished 
the second monadic
computation before you continue the first, this will not be a problem. 
But if you are forced
to insert a monadic value (not a return value) from the second 
activation into the first activation,
you have two different states, and you have to consider how to proceed. 
If you copy parts of the
second activation into the first state, you'll have problem when you 
leave the second and continue the
first activation. Thus, the only serious solution I can figure out at 
the moment is to ignore the
state of the second activation when inserting a value from the second 
one in the first one.

Coming back to the Q monad this would mean: ignore the name generation 
history from the
point where the expression to be spliced was *created* and take the name 
generation history
from the point where the splice is to be *inserted*, which is an older 
one. Anyway, the names
inside the generated splice take the newer information from the lexical 
scope of the creation.
However, I'm worried about that by turning an expression into a monadic 
form just by using
*return* tags this splice with a blank name history and I hope that the 
quasi quote implementation
will never proceed with this history but with the one from the lexical 
sope of the insertion point.

>In all other cases that
>typechecker should reject badly behaving programs.
>  
>
Unfortunately, I've had too many bad experiences which make me feel not 
that optimistic,
especially when working with state.

Best wishes
--
 Christoph