Add some extensions methods for validation

src/FSharpPlus/Data/Validation.fs

@@ -244,7 +244,6 @@ module Validation =
         coll1.Close (), coll2.Close ()
     #endif
 
-
 type Validation<'err,'a> with
 
     // as Applicative

src/FSharpPlus/Extensions/Validators.fs

@@ -0,0 +1,75 @@
+namespace FSharpPlus
+
+[<RequireQualifiedAccess>]
+module RequiredValidation =
+    open System
+    open FSharpPlus.Data
+
+    let inline validate error f v =
+        if f v then
+            Success v
+        else
+            Failure [ error ]
+
+    let inline string error value =
+        validate (error value) (String.IsNullOrWhiteSpace >> not) value
+
+    let inline greaterThan error min value =
+        validate (error value) (flip (>) min) value
+
+    let inline greaterOrEqualThan error min value =
+        validate (error value) (flip (>=) min) value
+
+    let inline email error value =
+        let check (v: string) =
+            try
+                let _ = Net.Mail.MailAddress(v)
+                true
+            with
+            | ex -> false
+
+        validate (error value) check value
+
+    let inline guid error value =
+        validate (error value) (fun v -> v <> Guid.Empty) value
+
+    let inline object error value =
+        let check value = box value <> null
+        validate (error value) check
+
+    let inline whenSome value checkWhenSome =
+        match value with
+        | Some v -> checkWhenSome v |> Validation.map Some
+        | _ -> Success None
+
+    let inline arrayValues values check =
+        let validated : Validation<_,_> [] =
+            values
+            |> Array.map check
+        validated
+        |> sequence
+        |> Validation.map Seq.toArray
+
+    let inline listValues values check =
+        let validated : List<Validation<_,_>> =
+            values
+            |> List.map check
+        validated
+        |> sequence
+        |> Validation.map Seq.toArray
+
+    let inline atLeastOne error =
+        let check values =
+            Seq.isEmpty values |> not
+
+        validate error check
+
+    let isSuccess =
+        function
+        | Success _ -> true
+        | Failure _ -> false
+
+    let isFailure =
+        function
+        | Success _ -> false
+        | Failure _ -> true

src/FSharpPlus/FSharpPlus.fsproj

@@ -102,6 +102,7 @@
     <Compile Include="Data/Coproduct.fs" />
     <Compile Include="Extensions/Observable.fs" />
     <Compile Include="Extensions/AsyncEnumerable.fs" />
+    <Compile Include="Extensions\Validators.fs" />
     <Compile Include="Memoization.fs" />
     <Compile Include="Parsing.fs" />
   </ItemGroup>

tests/FSharpPlus.Tests/Validations.fs

@@ -11,7 +11,17 @@ module Validation =
   open FSharpPlus.Data
   open Validation
   open FSharpPlus.Tests.Helpers
-
+
+  let private getSuccess =
+    function
+    | Success s -> s
+    | Failure _ -> failwith "It's a failure"
+
+  let private getFailure =
+    function
+    | Success _ -> failwith "It's a Success"
+    | Failure f -> f   
+
   let fsCheck s x = Check.One({Config.QuickThrowOnFailure with Name = s}, x)
   module FunctorP =
     [<Test>]
@@ -337,4 +347,53 @@ module Validation =
       let v: Validation<string Async, int Async> = Success (async {return 42})
       let r = Validation.bisequence v
       let subject = Async.RunSynchronously r
-      areStEqual subject (Success 42)
+      areStEqual subject (Success 42)
+
+    [<Test>]
+    [<TestCase("", false)>]
+    [<TestCase("   ", false)>]
+    [<TestCase(null, false)>]
+    [<TestCase("NotEmpty", true)>]
+    let testValidateRequireString (str, success) =
+      let error = konst "Str"
+      let r = RequiredValidation.string error str
+      areStEqual (RequiredValidation.isSuccess r) success
+
+      if not success then
+        let failure = getFailure r
+        areStEqual failure.Length 1
+        areStEqual failure.[0] (error "")
+      else
+        ()
+
+    [<Test>]
+    [<TestCase(1, 0, true)>]
+    [<TestCase(0, 0, false)>]
+    [<TestCase(-1, 0, false)>]
+    let testValidateRequireGreaterThan (value, limit, success) =
+      let error = konst "Int"
+      let r = RequiredValidation.greaterThan error limit value
+      areStEqual (RequiredValidation.isSuccess r) success
+
+      if not success then
+        let failure = getFailure r
+        areStEqual failure.Length 1
+        areStEqual failure.[0] (error 1)
+      else
+        ()
+
+    [<Test>]
+    [<TestCase(1, 0, true)>]
+    [<TestCase(0, 0, true)>]
+    [<TestCase(-1, 0, false)>]
+    let testValidateRequireGreaterOrEqualThan (value, limit, success) =
+      let error = konst "Int"
+      let r = RequiredValidation.greaterOrEqualThan error limit value
+      areStEqual (RequiredValidation.isSuccess r) success
+
+      if not success then
+        let failure = getFailure r
+        areStEqual failure.Length 1
+        areStEqual failure.[0] (error 1)
+      else
+        ()