如何从类型表示获取Data.Data.Constr优化Haskell持久化框架

Hey Thomas! Great question—let’s figure out how to ditch that TypeInfo parameter and let Haskell’s Data typeclass do the heavy lifting for your persistence framework.

The key insight here is that the Data a constraint already gives you all the type-level metadata you need, no sample entity or explicit TypeInfo required. Here’s how to make it work:

1. Use Data a to fetch the DataType without a sample value #

You don’t need a real instance of your entity to get its DataType—Haskell’s auto-generated Data instances implement dataTypeOf in a way that doesn’t require evaluating the value. You can safely use undefined :: a to trigger it:

import Data.Data

getEntityDataType :: forall a. Data a => DataType
getEntityDataType = dataTypeOf (undefined :: a)

Alternatively, if you prefer working with TypeRep (from Typeable), you can use dataTypeFromTypeRep to convert a type representation to a DataType:

import Data.Data
import Data.Typeable

getEntityDataType :: forall a. (Data a, Typeable a) => DataType
getEntityDataType = case dataTypeFromTypeRep (typeRep :: TypeRep a) of
    Just dt -> dt
    Nothing -> error "Entity type must have a Data instance"

2. Extract the constructor (Constr) from DataType #

Since your entities are record-style data types, they should have exactly one constructor. You can pull this from the DataType with dataTypeConstrs:

getEntityConstr :: forall a. Data a => Constr
getEntityConstr = case dataTypeConstrs getEntityDataType of
    [constr] -> constr
    _ -> error "Entities must be single-constructor record types"

This enforces your framework’s expectation that entities are simple records—you can adjust the error handling if you need to support multi-constructor types later.

3. Pull record field metadata from the Constr #

Once you have the constructor, constrFields gives you the list of record field names (perfect for mapping to database columns or serialization keys):

getEntityFields :: forall a. Data a => [String]
getEntityFields = constrFields getEntityConstr

4. Simplify your retrieveEntityById signature #

Now you can rewrite your function to rely solely on Data a (and Typeable a if needed) instead of requiring TypeInfo:

retrieveEntityById :: (Data a, Typeable a) => Id -> IO (Maybe a)
retrieveEntityById entityId = do
    -- Fetch the metadata we need
    let dt = getEntityDataType @a
        constr = getEntityConstr @a
        fields = getEntityFields @a
    
    -- Use dt, constr, and fields to implement your persistence logic here
    -- (e.g., generate a database query using the field names, deserialize rows using the constructor)
    ...

Why your earlier attempts with Proxy/TypeRep didn’t work #

You were on the right track—you just missed that TypeRep alone isn’t enough to get Constr or field info. You need to bridge TypeRep to DataType (via dataTypeFromTypeRep) or use Data a directly to access the full type metadata that includes constructor and field details.

Bonus: Avoid undefined if you’re squeamish #

If you don’t want to use undefined, you can use a Proxy to carry the type information instead:

getEntityDataType :: forall a. Data a => Proxy a -> DataType
getEntityDataType _ = dataTypeOf (undefined :: a)

Then call it as getEntityDataType (Proxy :: Proxy Person)—but since you can already get the type from the function’s a parameter, this is mostly a stylistic choice.

内容的提问来源于stack exchange，提问作者Thomas Mahler