Lists of data types: "could not deduce (a ~ SomeType) from the context (SomeTypeclass a)"

the offending line is stList :: (Show a) => [a]. You're declaring that stList is a polymorphic list that holds any element which satisfies the show constraint. But stList isn't a polymorphic list! It's a list of SampleTypes. So remove the signature and see what ghci infers, or just give it the correct signature: :: [SampleType].

stList :: (Show a) => [a]

is saying that given any Show instance for any type a, you are going return a list of elements of that type.

stList = [SampleTypeConstructor]

is returning list of SampleTypes, while that is a list of elements for which there exists a Show instance, it is not a list that works for every choice of a.

In reality the only inhabitant you are likely to find for this type that doesn't involve bottoms is [], because Show a doesn't provide any mechanism to construct an a.

To fix this you can do one of a few things, depending on your end goal.

You may just want to let stList have the more narrow type:

stList :: [SampleType]

You may want to build some kind of type like

newtype Showable = Showable (Int -> String -> String)

which explicitly captures the relevant portion of the Show instance. (You can also do this with an existential type, but this version is Haskell 98.)

instance Show Showable where
    showsPrec d (Showable f) = f d

showable :: Show a => a -> Showable
showable a = Showable (\d -> showsPrec d a)

Then you could make a list of Showables.

stList :: [Showable]
stList = [showable SampleTypeConstructor]

But ultimately it depends on what you are trying to accomplish.