Dear all, I wrote the following  types:

> class Transformation t where
>  (<+>) :: t -> SPLModel  -> InstanceModel -> InstanceModel

> data Configuration = forall t . Transformation t => Configuration (FeatureExpression, [t])
> type ConfigurationKnowledge = [Configuration]

I tried to write a function that retrieves the list of transformations of a configuration. Bellow a code snip of such a function.

> transformations ck fc = concat [snd c | (Configuration c) <- ck, eval fc (fst c)]

However, compiling this I got:

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Inferred type is less polymorphic than expected
Quantified type variable `t' escapes
When checking an existential match that binds
c :: (FeatureModel.Types.FeatureExpression, [t])
The pattern(s) have type(s): Configuration
The body has type: [t]
In a stmt of a list comprehension: (Configuration c) <- ck
In the first argument of `concat', namely
`[snd c | (Configuration c) <- ck, eval fc (fst c)]'

---

How can I fix this problem?

Thanks,

Rodrigo.