Haskell fromList使用难题：函数类型不符预期（附代码链接）

Hey there! Let's work through this type mismatch issue you're hitting with Haskell's fromList.

The core problem here is that somewhere in your code, you're wrapping the Map Role [Permission] part in an extra list ([]), which is why you're getting ([Permission], [Map Role [Permission]]) instead of the desired ([Permission], Map Role [Permission]) for each entry in your Map User ....

How to Fix It #

Let's break this down with concrete examples to clarify:

1. Check your entry structure
   You likely have code that looks like this (the problematic version):

   -- This creates the unwanted [Map Role [Permission]] type
   let userEntry = (myUser, (myPermissions, [myRoleMap]))
   let myMap = fromList [userEntry]
   

   Instead, remove the extra brackets around myRoleMap to get the correct type:

   -- This gives you the target (Permissions, Map Role [Permission]) type
   let userEntry = (myUser, (myPermissions, myRoleMap))
   let myMap = fromList [userEntry]
   

2. If coming from an aggregation step
   If you're generating these entries from a fold, group, or other aggregation, you might be accidentally collecting a single Map Role [Permission] into a list. For example, if groupBy leaves you with a list containing one map, extract it safely (preferably with a safe function instead of head to avoid runtime errors):

   -- Safe helper to extract a single map from a list
   extractSingleMap :: [Map k v] -> Maybe (Map k v)
   extractSingleMap [m] = Just m
   extractSingleMap _ = Nothing
   
   -- Use it when building your entry
   case extractSingleMap roleMapsList of
     Just roleMap -> (myUser, (myPermissions, roleMap))
     Nothing -> -- handle empty/multiple map cases appropriately
   

Verify the Input Type #

Double-check the type of the list you're passing to fromList by running :t myList in GHCi. It should match [(User, ([Permission], Map Role [Permission]))]—this ensures fromList will produce the exact Map User ([Permission], Map Role [Permission]) you need.

内容的提问来源于stack exchange，提问作者Alexia Desouza