CODEWITHSUNDEEP
haskellyesod
YHVH
YHVH

Reputation: 585

Model validation in Yesod

I'd like to define a constraint on a field so reserved words are not valid. How can I accomplish this in Yesod?

{-# LANGUAGE TemplateHaskell #-}
module Sid where

import Control.Exception
import Data.List
import Database.Persist.TH
import Data.Aeson.TH
import Text.Show
import Text.Read
import Data.Text

data Sid = Sid Text
         deriving (Show, Read)

reserved :: [Text]
reserved = ["abc","def"]

allowedSid :: Text -> Sid
allowedSid a = (assert (notElem a reserved)) Sid a

derivePersistField "allowedSid"
deriveJSON defaultOptions ''allowedSid

Upvotes: 4

Views: 156

Answers (1)

Michael Snoyman
Michael Snoyman

Reputation: 31315

I'd do this in normal Haskell instead: create a newtype wrapper with some smart constructors. This would look something like:

newtype Sid = Sid Text

mkSid :: Text -> Maybe Sid
mkSid t = if t `elem` reserved then Nothing else Just t

Put this code in a module, and don't export the Sid data constructor.

Upvotes: 4

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