get persistent's field definitions for my model

Question

given a persistent model definition like this:

mkPersist sqlSettings [persistLowerCase|
Person
    name String
    age Int
    deriving Show
|]

I'm looking for a way to get the field definitions for this model. FieldDef looks like a promising datatype, but persistent does not generate a getPersonFields :: [FieldDef] function, so how can it be done?

In case this is a XY problem - here's the background: I want to return some kind of metadata in case of json parse errors. If I try to POST a new person but the json is wrong, I want to respond with something like:

{
    "result": "error",
    "code": 8,
    "message": "Could not parse payload as person",
    "fields": [
        {
            "name": "name",
            "type": "string"
        },
        {
            "name": "age",
            "type": "int"
        }
    ]
}

Answer 1

A sample example showing how it is done:

#!/usr/bin/env stack
{- stack
     --resolver lts-9.0
     --install-ghc
     runghc
     --package persistent
     --package persistent-sqlite
     --package persistent-template
-}

{-# LANGUAGE EmptyDataDecls             #-}
{-# LANGUAGE FlexibleContexts           #-}
{-# LANGUAGE GADTs                      #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE MultiParamTypeClasses      #-}
{-# LANGUAGE OverloadedStrings          #-}
{-# LANGUAGE QuasiQuotes                #-}
{-# LANGUAGE TemplateHaskell            #-}
{-# LANGUAGE TypeFamilies               #-}
import           Control.Monad.IO.Class  (liftIO)
import           Database.Persist
import           Database.Persist.Sqlite
import           Database.Persist.TH

share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persistLowerCase|
Person
    name String
    age Int Maybe
    deriving Show
BlogPost
    title String
    authorId PersonId
    deriving Show
|]

main :: IO ()
main = let efields = entityFields $ entityDef (undefined :: Maybe Person)
       in print efields

Demo:

$ stack efield.hs
[FieldDef {fieldHaskell = HaskellName {unHaskellName = "name"}, fieldDB = DBName {unDBName = "name"}, fieldType = FTTypeCon Nothing "String", fieldSqlType = SqlString, fieldAttrs = [], fieldStrict = True, fieldReference = NoReference},FieldDef {fieldHaskell = HaskellName {unHaskellName = "age"}, fieldDB = DBName {unDBName = "age"}, fieldType = FTTypeCon Nothing "Int", fieldSqlType = SqlInt64, fieldAttrs = ["Maybe"], fieldStrict = True, fieldReference = NoReference}]