forma
Parse and validate forms in JSON format
https://github.com/mrkkrp/forma
Version on this page: | 1.1.1 |
LTS Haskell 22.37: | 1.2.0@rev:3 |
Stackage Nightly 2024-10-11: | 1.2.0@rev:3 |
Latest on Hackage: | 1.2.0@rev:3 |
forma-1.1.1@sha256:0285515953f9502b7567d79b617d8b07a828aa87cec3b567c46d2774c18dc517,2291
Module documentation for 1.1.1
- Web
Forma
This module provides a tool for validation of forms that are represented in the JSON format. Sending forms in JSON format via an AJAX request instead of traditional submitting of forms has a number of advantages:
-
Smoother user experience: no need to reload the whole page.
-
Form rendering is separated and lives only in GET handler, POST (or whatever method you deem appropriate for your use case) handler only handles validation and actual effects that form submission should initiate.
-
You get a chance to organize form input just like you want.
The task of validation of a form in the JSON format may seem simple, but it’s not trivial to get it right. The library allows you to:
-
Define form parser using type-safe applicative notation with field labels being stored on the type label which guards against typos and will force all your field labels be always up to date.
-
Parse JSON
Value
according to the definition of form you created. -
Stop parsing immediately if given form is malformed and cannot be processed.
-
Validate forms using any number of composable checkers that you write for your specific problem domain. Once you have a vocabulary of checkers, creation of new forms is just a matter of combining them, and yes they do combine nicely.
-
Collect validation errors from multiple branches of parsing (one branch per form field) in parallel, so validation errors in one branch do not prevent us from collecting validation errors from other branches. This allows for a better user experience as the user can see all validation errors at the same time.
-
Use
optional
and(<|>)
fromControl.Applicative
in your form definitions instead of ugly ad-hoc stuff. -
Perform validation using several form fields at once. You choose which “sub-region” of your form a given check will have access to, see
withCheck
.
Example of use
Here is a complete working example:
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE OverloadedLabels #-}
{-# LANGUAGE OverloadedStrings #-}
module Main (main) where
import Control.Monad.Except
import Data.Aeson
import Data.Text (Text)
import Web.Forma
import qualified Data.Text as T
type LoginFields = '["username", "password", "remember_me"]
data LoginForm = LoginForm
{ loginUsername :: Text
, loginPassword :: Text
, loginRememberMe :: Bool
} deriving (Show)
loginForm :: Monad m => FormParser LoginFields Text m LoginForm
loginForm = LoginForm
<$> field #username notEmpty
<*> field #password notEmpty
<*> field' #remember_me
notEmpty :: Monad m => Text -> ExceptT Text m Text
notEmpty txt =
if T.null txt
then throwError "This field cannot be empty."
else return txt
myInput :: Value
myInput = object
[ "username" .= ("Bob" :: Text)
, "password" .= ("123" :: Text)
, "remember_me" .= True
]
main :: IO ()
main = runForm loginForm myInput >>= print
You may want to play with it a bit before writing serious code.
Contribution
Issues, bugs, and questions may be reported in the GitHub issue tracker for this project.
Pull requests are also welcome and will be reviewed quickly.
License
Copyright © 2017–2019 Mark Karpov
Distributed under BSD 3 clause license.
Changes
Forma 1.1.1
- Fixed a bug which caused
withCheck
(and functions using it such asfield
) report incorrect location of element for which validation fails when it’s nested insubParser
wrappers.
Forma 1.1.0
- Added
runFormPure
.
Forma 1.0.0
- The library has been completely redesigned and rewritten.
Forma 0.2.0
- Added
withCheck
.
Forma 0.1.0
- Initial release.