[Haskell-cafe] Help using Groundhog Template Haskell.

Andrew Myers asm198 at gmail.com
Fri Nov 8 02:38:19 UTC 2013

Hi Cafe,

I'm trying to experiment with groundhog because I want something that I can map between arbitrary ADTs and arbitrary Schema.
Groundhog looks like exactly what I want but I'm hung up on an error coming from the mkPersist TH function.  When I compile
the code below with ghc --make groundhog.hs I get this error:

    Function binding for `Database.Groundhog.Core.entityFieldChain' has no equations
    When splicing a TH declaration:

I'm pretty sure it has something to do with the TransactionType not being made up of primitive types but I'm not sure
what to do about it.  Can someone familiar with the library point me in the right direction?

Andrew Myers

{-# LANGUAGE TemplateHaskell, QuasiQuotes #-}
module Checking.Database.Groundhog where

import qualified Database.Groundhog.TH as TH
import qualified Data.Text as T
import qualified Data.Time as D
import qualified Control.Lens as L

data TransactionType = Deposit | Debt | Informational
  deriving (Show, Eq, Read)

data Transaction = Transaction {
    _transactionDate :: D.UTCTime
  , _transactionAmount :: Double
  , _transactionDescription :: T.Text
  , _transactionBalanced :: Bool
  , _transactionCheckNumber :: Maybe Int
  , _transactionTransactionType :: TransactionType
  deriving (Show, Read, Eq)

TH.mkPersist TH.defaultCodegenConfig [TH.groundhog|
- entity: Transaction
    - name: Transaction
        - name: _transactionDate
          dbName: date
          exprName: DateField
        - name: _transactionAmount
          dbName: amount
          exprName: AmountField
        - name: _transactionDescription
          dbName: description
          exprName: DescriptionField
        - name: _transactionBalanced
          dbName: balanced
          exprName: BalancedField
        - name: _transactionCheckNumber
          dbName: check_number
          exprName: CheckNumberField
        - name: _transactionTransactionType
          dbName: transaction_type
          exprName: TransactionTypeField

- entity: TransactionType

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