gistfile1.txt

src/MinimalProjectGroups.hs:27:25: error:
    • Couldn't match type ‘skill1’ with ‘skill’
      ‘skill1’ is a rigid type variable bound by
        the type signature for:
          skillsOf :: forall skill1.
                      (Ord employee, Ord skill1) =>
                      employee -> Set skill1
        at src/MinimalProjectGroups.hs:26:5-66
      ‘skill’ is a rigid type variable bound by
        the type signature for:
          minimalGroups :: forall employee skill.
                           (Ord employee, Ord skill) =>
                           Map employee (Set skill) -> Set skill -> Set (Set employee)
        at src/MinimalProjectGroups.hs:(13,1)-(15,64)
      Expected type: Set skill1
        Actual type: Set skill
    • In the expression: Map.findWithDefault Set.empty employee staff
      In an equation for ‘skillsOf’:
          skillsOf employee = Map.findWithDefault Set.empty employee staff
      In an equation for ‘minimalGroups’:
          minimalGroups staff requirements
            = Set.filter
                (\ group -> isSufficient group && isMinimal group) allGroups
            where
                allGroups :: Ord employee => Set (Set employee)
                allGroups = Set.powerSet (Map.keysSet staff)
                isSufficient :: Ord employee => Set employee -> Bool
                isSufficient group
                  = requirements `Set.isSubsetOf` Set.unions (Set.map skillsOf group)
                ....
    • Relevant bindings include
        skillsOf :: employee -> Set skill1
          (bound at src/MinimalProjectGroups.hs:27:5)
        requirements :: Set skill
          (bound at src/MinimalProjectGroups.hs:16:21)
        staff :: Map employee (Set skill)
          (bound at src/MinimalProjectGroups.hs:16:15)
        minimalGroups :: Map employee (Set skill)
                         -> Set skill -> Set (Set employee)
          (bound at src/MinimalProjectGroups.hs:16:1)
   |
27 |     skillsOf employee = Map.findWithDefault Set.empty employee staff
   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

MinimalGroups.hs

{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}

module MinimalProjectGroups where

import Data.Map.Strict (Map)
import qualified Data.Map.Strict as Map
import Data.Set (Set)
import qualified Data.Set as Set
import Data.Text (Text)
import qualified Data.Text as Text

minimalGroups
  :: forall employee. forall skill. (Ord employee, Ord skill)
  => Map employee (Set skill) -> Set skill -> Set (Set employee)
minimalGroups staff requirements =
  Set.filter (\group -> isSufficient group && isMinimal group) allGroups
  where
    allGroups :: Ord employee => Set (Set employee)
    allGroups = Set.powerSet (Map.keysSet staff)

    isSufficient :: Ord employee => Set employee -> Bool
    isSufficient group =
      requirements `Set.isSubsetOf` Set.unions (Set.map skillsOf group)

    skillsOf :: (Ord employee, Ord skill) => employee -> Set skill
    skillsOf employee = Map.findWithDefault Set.empty employee staff

    isMinimal :: Ord employee => Set employee -> Bool
    isMinimal group = all (`isEssentialFor` group) group

    isEssentialFor :: Ord employee => employee -> Set employee -> Bool
    isEssentialFor employee group =
      not (isSufficient (Set.delete employee group))