{"id":11922,"date":"2014-08-26T15:06:00","date_gmt":"2014-08-26T14:06:00","guid":{"rendered":"http:\/\/touk.pl\/blog\/?guid=9ffb9caa0030a17a5cbd9bd493416443"},"modified":"2015-08-18T13:11:21","modified_gmt":"2015-08-18T12:11:21","slug":"validation-with-warnings-in-scala-with-scalaz","status":"publish","type":"post","link":"https:\/\/touk.pl\/blog\/2014\/08\/26\/validation-with-warnings-in-scala-with-scalaz\/","title":{"rendered":"Validation with warnings in scala with scalaz"},"content":{"rendered":"

Monad<\/b> are containers with ‘special powers’, when it comes to applying function over its content.
Validation special power is propagating Failure over validation process.<\/p>\n

If you are not familiar with scalaz.Validation I urge you to read this example, which shows how to use Validation: A Tale of 3 Nightclubs<\/a><\/p>\n

Basically validation looks like this:<\/p>\n

Scalaz.Validation uses idiomatic scala way to compose monads by For Comprehension<\/b>.<\/p>\n

Concrete validation method, returning scalaz.Validation instances looks like this:<\/p>\n

Scalaz provide helper methods for wrapping values into Failure or Success.<\/p>\n

To sum it up. Validation is a an elegant way to handle application validation logic.<\/p>\n

However it’s not enough. <\/h1>\n

Our business rules require application logic’s to perform validation with warnings, which should not propagate as failures, but rather propagate independently of Success\/Failure types.<\/p>\n

We liked monad approach to data validation so we wanted to keep it that way.<\/p>\n

Let me introduce Validation with warnings<\/b><\/p>\n

What it does is basically wrapping scalaz.Validation into another type responsible for carrying warnings over validation process<\/p>\n

Thank to scala type inference our validation code look’s just the same, but now for expression operates on ValidationWithWarnings type rather than Validation.<\/p>\n

OK, but what about validation code? We created similar helper methods for wrapping validation into ValidationWithWarnings and wrapping values directly into warnings.<\/p>\n

One could inline warning in for loop:<\/p>\n

Or use it in validation method:<\/p>\n

And of course chain it in for-loop:<\/p>\n

<\/p>\n

Applicative <\/h3>\n
We support scalaz.Applicative, so it’s possible to take few validations and apply them to function if all elements are successes, collecting any errors and warnings if present. <\/p>\n

<\/p>\n

\n

Summing <\/h3>\n<\/div>\n

Similarly to scalaz.Validation, we also support summing values, if value type has Semigroup typeclass:<\/p>\n

Repository <\/h3>\n<\/div>\n
Code with examples in test files can be found at https:\/\/github.com\/Ajk4\/ValidationWithWarnings <\/a><\/div>\n

<\/p>\n

\n

Q&A<\/h3>\n<\/div>\n
Why not use Writer Monad?<\/b>
– Same reason why we prefer Validation over Either with left\/right projection. It’s more direct and descriptive.<\/p>\n

Why validation nel underhood?<\/b>
– It suited our business needs best.<\/p>\n

Validation is not a Monad!<\/b>
– True. <\/div>\n","protected":false},"excerpt":{"rendered":"Monad are containers with ‘special powers’, when it comes to applying function over its content.Validation special power is propagating Failure over validation process.If you are not familiar with scalaz.Validation I urge you to read this example,…\n","protected":false},"author":51,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[11],"tags":[113],"class_list":{"0":"post-11922","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-development-design","7":"tag-scala"},"_links":{"self":[{"href":"https:\/\/touk.pl\/blog\/wp-json\/wp\/v2\/posts\/11922","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/touk.pl\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/touk.pl\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/touk.pl\/blog\/wp-json\/wp\/v2\/users\/51"}],"replies":[{"embeddable":true,"href":"https:\/\/touk.pl\/blog\/wp-json\/wp\/v2\/comments?post=11922"}],"version-history":[{"count":3,"href":"https:\/\/touk.pl\/blog\/wp-json\/wp\/v2\/posts\/11922\/revisions"}],"predecessor-version":[{"id":12418,"href":"https:\/\/touk.pl\/blog\/wp-json\/wp\/v2\/posts\/11922\/revisions\/12418"}],"wp:attachment":[{"href":"https:\/\/touk.pl\/blog\/wp-json\/wp\/v2\/media?parent=11922"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/touk.pl\/blog\/wp-json\/wp\/v2\/categories?post=11922"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/touk.pl\/blog\/wp-json\/wp\/v2\/tags?post=11922"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}