Add some extensions methods for validation

src/FSharpPlus/Data/Validation.fs

@@ -243,6 +243,29 @@ module Validation =
         List.iter (function Success e -> coll1.Add e | Failure e -> coll2.Add e) source
         coll1.Close (), coll2.Close ()
     #endif
+
+    [<AutoOpen>]
+    module ComputationExpression =
+        type ValidationBuilder() =
+            member _.Bind(x, f) =
+                match x with
+                | Failure e -> Failure e
+                | Success a -> f a
+
+            member _.MergeSources(left : Validation<list<string>, _>, right : Validation<list<string>, _>) =
+                match left, right with
+                | Success l, Success r -> Success (l, r)
+                | Failure l, Success _ -> Failure l
+                | Success _, Failure r -> Failure r
+                | Failure r, Failure l -> List.append r l |> Failure
+
+            member _.Return(x) =
+                Success x
+
+            member _.ReturnFrom(x) =
+                x
+
+        let validator = ValidationBuilder()
 
 
 type Validation<'err,'a> with

src/FSharpPlus/Extensions/Validation.fs

@@ -0,0 +1,80 @@
+namespace FSharpPlus
+
+[<RequireQualifiedAccess>]
+module Validations =
+    open System
+    open FSharpPlus.Data
+
+    let inline validate errorMessage f v =
+        if f v then
+            Success v
+        else
+            Failure [ errorMessage ]
+
+    let inline requireString propName =
+        let errorMessage =
+            sprintf "%s cannot be null, empty or whitespace." propName
+
+        validate errorMessage (String.IsNullOrWhiteSpace >> not)
+
+    let inline requireGreaterThan propName min =
+        let errorMessage =
+            sprintf "%s have to be greater or equal to '%d'." propName min
+
+        validate errorMessage (flip (>) min)
+
+    let inline requireGreaterOrEqualThan propName min =
+        let errorMessage =
+            sprintf "%s have to be greater or equal to '%d'." propName min
+
+        validate errorMessage (flip (>=) min)
+
+    let inline requireEmail propName =
+        let errorMessage =
+            sprintf "%s is not a valid email address." propName
+
+        let check (v: string) =
+            try
+                let _ = Net.Mail.MailAddress(v)
+                true
+            with
+            | ex -> false
+
+        validate errorMessage check
+
+    let inline requireGuid propName =
+        validate (sprintf "%s is required" propName) (fun v -> v <> Guid.Empty)
+
+    let inline requireObject propName =
+        let check value = box value <> null
+        validate (sprintf "%s is required" propName) check
+
+    let inline requireWhenSome value checkWhenSome =
+        match value with
+        | Some v -> checkWhenSome v |> Validation.map Some
+        | _ -> Success None
+
+    let inline requireArrayValues values check =
+        let validated : Validation<_,_> [] =
+            values
+            |> Array.map check
+        validated
+        |> sequence
+        |> Validation.map Seq.toArray
+
+    let inline requireListValues values check =
+        let validated : List<Validation<_,_>> =
+            values
+            |> List.map check
+        validated
+        |> sequence
+        |> Validation.map Seq.toArray
+
+    let inline requireAtLeastOne propName =
+        let check values =
+            Seq.isEmpty values |> not
+
+        let errorMessage =
+            sprintf "%s should have at least one element'." propName
+
+        validate errorMessage check

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/Validation.fs" />
     <Compile Include="Memoization.fs" />
     <Compile Include="Parsing.fs" />
   </ItemGroup>

tests/FSharpPlus.Tests/Validations.fs

@@ -11,7 +11,27 @@ module Validation =
   open FSharpPlus.Data
   open Validation
   open FSharpPlus.Tests.Helpers
-
+
+  let private isSuccess =
+    function
+    | Success _ -> true
+    | Failure _ -> false
+
+  let private isFailure =
+    function
+    | Success _ -> false
+    | Failure _ -> true
+
+  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 +357,50 @@ 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 r = Validations.requireString "Str" str
+      areStEqual (isSuccess r) success
+
+      if not success then
+        let failure = getFailure r
+        areStEqual failure.Length 1
+      else
+        ()
+
+    [<Test>]
+    [<TestCase(1, 0, true)>]
+    [<TestCase(0, 0, false)>]
+    [<TestCase(-1, 0, false)>]
+    let testValidateRequireGreaterThan (value, limit, success) =
+
+      let r = Validations.requireGreaterThan "Int" limit value
+      areStEqual (isSuccess r) success
+
+      if not success then
+        let failure = getFailure r
+        areStEqual failure.Length 1
+      else
+        ()
+
+    [<Test>]
+    [<TestCase(1, 0, true)>]
+    [<TestCase(0, 0, true)>]
+    [<TestCase(-1, 0, false)>]
+    let testValidateRequireGreaterOrEqualThan (value, limit, success) =
+
+      let r = Validations.requireGreaterOrEqualThan "Int" limit value
+      areStEqual (isSuccess r) success
+
+      if not success then
+        let failure = getFailure r
+        areStEqual failure.Length 1
+      else
+        ()