sketch of what I was saying on the call:

type family RuleInput k :: Type

newtype ProjectHaskellFile = ProjectHaskellFile NormalizedFilePath

-- examples
type instance RuleInput GhcSession = ProjectHaskellFile
type instance RuleInput GetClientSettings = ()

-- various stuff might require `RuleInputFromFilePath (RuleInput r)` 
-- which basically says you have a way to convert a filepath into
-- input for the rule, possibly failing
class RuleInputFromFilePath i where
   toRuleInput :: NormalizedFilePath -> Either String i

-- example
someRule = do
   ...
   validFp <- case toRuleInput nfp of
     Right p -> pure p
     Left e -> ... some error ...
   use someRule validFp

sketch of what I was saying on the call:

type family RuleInput k :: Type

newtype ProjectHaskellFile = ProjectHaskellFile NormalizedFilePath

-- examples
type instance RuleInput GhcSession = ProjectHaskellFile
type instance RuleInput GetClientSettings = ()

-- various stuff might require `RuleInputFromFilePath (RuleInput r)` 
-- which basically says you have a way to convert a filepath into
-- input for the rule, possibly failing
class RuleInputFromFilePath i where
   toRuleInput :: NormalizedFilePath -> Either String i

-- example
someRule = do
   ...
   validFp <- case toRuleInput nfp of
     Right p -> pure p
     Left e -> ... some error ...
   use someRule validFp

@michaelpj The problem with that approach is that for some rules, both project haskell files and dependency haskell files are valid inputs. This is why I distinguish between the ValidInputs kind and the InputType kind.

The problem with that approach is that for some rules, both project haskell files and dependency haskell files are valid inputs.

Sure, so then you would need to have ProjectOrHaskellFile as the rule input and your RuleInputFromFilePath would be a bit more complex. It doesn't solve the "open universe of input file types" problem, but I think it's maybe a little nicer than what you're doing right now, and does solve the "this rule should take an input of type ()" problem.