Add README.md

Author: paluh

README.md

@@ -0,0 +1,51 @@
+# purescript-polyform-validators
+
+Set of useful validators which are build on top of `Validation` from purescript-polyform.
+
+## Usage
+
+`Validation` type from polyform has Applicative and Category instance. Using these two interfaces and provided validators you can quite easily write whole validation chain: from ajax request to resulting object. This validation flow will aggregate in typed manner all encoutered errors at given failing step.
+
+Let's look at real example - here we are making a request to shutterstock API and validating its response. `Data.Record.Fold.collect` is like sequence but over record (so it takes an record of `Validation`s and returns `Validation` into record):
+
+``` purescript
+type SearchResult image =
+  { page ∷ Int
+  , perPage ∷  Int
+  , totalCount ∷ Int
+  , searchId ∷ String
+  , photos ∷ Array image
+  }
+
+searchResult
+  :: forall err m
+   . Monad m
+  => Validation m
+      (Array (Variant (JsError err)))
+      Json
+      (SearchResult Image)
+searchResult = collect
+  { page: field "page" int
+  , perPage: field "per_page" int
+  , totalCount: field "total_count" int
+  , searchId: field "search_id" string
+  , photos: field "data" $ arrayOf image
+  }
+```
+
+Now using `affjaxJson` validator from this library we can chain these two validators to get fully validated request:
+
+``` purescript
+searchValidation
+  :: forall ext err
+   . Validation
+      ( Aff( ajax :: AJAX| ext))
+      (Array(Variant(SearchErrorRow err)))
+      (AffjaxRequest Unit)
+      (SearchResult Image)
+searchValidation = searchResult <<< affjaxJson
+```
+
+## Examples
+
+For more usage examples please refer these simple libraries: [purescript-shutterstock](https://github.com/lambdaterms/purescript-shutterstock), [purescript-pexels](https://github.com/lambdaterms/purescript-pexels) or [purescript-nasa-images](https://github.com/lambdaterms/purescript-nasa-images).