Haskell PostgreSQL Simple FromField

Question

I'm having trouble with defining FromField instance for my custom enum data type:

data Role = Staff | Student deriving (Eq, Ord, Show)

To represent Role in my database, I will use ints 0 and 1 (more roles could be added later) most likely smallint.

Review is used in my other data type that represents a row in my db table:

data ReviewAssignment = ReviewAssignment
  { aID :: ID
  , reviewID :: ID
  , reviewerID :: ID
  , revieweeID :: ID
  , reviewerRole :: Role -- <-- right there
  , reviewFinished :: Bool
  , reviewPoints :: Float
  } deriving (Show, Eq)

This is my FromRow instance for ReviewAssignment:

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

Now I'm just missing the following which I don't understand how to implement :

instance FromField Role where
  fromField = undefined

score 2 · Accepted Answer · answered Jul 18 '16 at 06:31

2

Probably to late but something like this should work (hopefully):

instance FromField Role where
  fromField f mdata =
    return role
    where role = case mdata of
      Just 0 -> Staff   
      _      -> Student

answered Jul 18 '16 at 06:31

itsu

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Emil Vikström · Answer 2 · 2016-01-21T10:04:36.693

1

fromField should take an integer (the smallint from your database) and return a Role value. I sugegst you implement it using pattern matching. You can read more about fromField in the documentation. You can also take inspiration from predefined fromField implementations.

edited Jan 21 '16 at 10:04

answered Jan 21 '16 at 09:42

Emil Vikström

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I have read the documentation, but I still don't understand it quite clearly so I'm asking here. I do understand the concept, but I don't know how to do it with FromField – Reygoch Jan 21 '16 at 09:45

Haskell PostgreSQL Simple FromField

2 Answers2