CODEWITHSUNDEEP
haskellpostgresql-simple
Tae
Tae

Reputation: 1695

‘toRow’ is not a (visible) method of class ‘ToRow’

I'm learning Haskell so I decided to write a web app. I did choose PostgreSQL Simple to work with the database. I successfully connected to it and tried simple math operations but I'm having problems trying to map records to data. This code doesn't compile:

module Handlers.SurveyReplies where

import Database.PostgreSQL.Simple

data AnswersSet = AnswersSet {
  sex ∷ Integer,
  ageRange ∷ Integer,
  country ∷ Integer,
  commune ∷ Maybe Integer
} deriving (Show)

instance FromRow AnswersSet where
  fromRow = AnswersSet <$> field <*> field <*> field <*> field

instance ToRow AnswersSet where
toRow r = [toField (sex r), toField (ageRange r), toField (country r), toField (commune r)]

The error is:

    ‘fromRow’ is not a (visible) method of class ‘FromRow’
   |
17 |   fromRow = AnswersSet <$> field <*> field <*> field <*> field
   |   ^^^^^^^

And also:

    ‘toRow’ is not a (visible) method of class ‘ToRow’
   |
20 |         toRow r = [toField (sex r), toField (ageRange r), toField (country r), toField (commune r)]
   |         ^^^^^

I looked some sample projects (this among others) but I don't get what I'm doing wrong :(

Upvotes: 1

Views: 1002

Answers (1)

Mor A.
Mor A.

Reputation: 4626

The module Database.PostgreSQL.Simple only exports the type classes ToRow and FromRow, without any of their methods.

For those methods you need to import the modules Database.PostgreSQL.Simple.ToRow and Database.PostgreSQL.Simple.FromRow, as is done in the sample you linked to.

Upvotes: 7

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