On Tue, Dec 1, 2009 at 1:00 PM, rodrigo.bonifacio
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:
--- 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?
The problem is that transformations is creating a heterogenous list,
i.e., there is no guarantee that the contents of the list all have the
same type.
You can get around this by declaring a type representing any transformation,
data SomeTransformation = forall t. Transformation t => ST t
and having transformation return a list of those.
However, if Transformation really only has one member function, you'd
be better off eliminating the existential types entirely.
e.g.,
data Configuration = Configuration FeatureExpression (SPLModel ->
InstanceModel -> InstanceModel)
--
Dave Menendez