> > I'm passing that 'record' around as an implicit value. The record as
> > STRefs that I use to collect info, but it also has some pure fields with
> > 'read-only' info. Something like,
<opss let me change 'max' to 'm'>
> > data E s = E{
> > refi :: STRef s Int,
> > refc :: STRef s Char,
> > m :: Int
> > }
> >
> > In some functions I might need only some pure fields, and none of the
> > STRefs, but since I pass something of type 'E s' around, everything ends
> > up beeing monadic.
>
> there is no reason why that should be the case. you only need to be
> within the ST monad when you read the references, so if in some function
> you only look at the pure values, it can have a pure type.
> hope this helped
> iavor
That's what I thought, untill I tried it.
Well, actually you must be right since the pure field defines a pure
(projection) function... Hmmm, ok, can someone explain this to me,
data E s = E{
refi :: STRef s Int,
refc :: STRef s Char,
m :: Int
}
-- this is fine, obviously...
pure :: E s -> Int
pure e = m e
-- but this is not...
pure2 :: (?e :: E s) => Int
pure2 = m (?e)
Why exactly isn't this allowed? What is the workaround?
Error msg:
------------------------------------------------------------------------
Ambiguous constraint `?e :: E s'
At least one of the forall'd type variables mentioned by the
constraint
must be reachable from the type after the '=>'
In the type: forall s. (?e :: E s) => Int
While checking the type signature for `pure2'
Failed, modules loaded: none.
--------------------------------------------------------------------------
Thanks,
J.A.
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