added validation for registration. didn't test it

src/Register.elm

@@ -4,6 +4,7 @@ import Browser exposing (..)
 import Html exposing (..)
 import Html.Attributes exposing (..)
 import Html.Events exposing (..)
+import List exposing (..)
 import Route exposing (..)
 
 
@@ -18,7 +19,7 @@ type alias Model =
 
 
 type Problem
-    = InvalidEntry String
+    = InvalidEntry ValidatedField String
     | ServerError String
 
 
@@ -69,6 +70,18 @@ update msg model =
         EnteredName name ->
             updateForm (\form -> { form | name = name }) model
 
+        SubmittedForm ->
+            case validate model.form of
+                Ok validForm ->
+                    ( { model | problems = [] }
+                    , Cmd.none
+                    )
+
+                Err problems ->
+                    ( { model | problems = problems }
+                    , Cmd.none
+                    )
+
         _ ->
             ( model, Cmd.none )
 
@@ -135,11 +148,122 @@ view model =
                         [ a [ Route.href Route.Login ]
                             [ text "Already have an account?" ]
                         ]
-
-                    -- , ul [ class "error-messages" ]
-                    --     (List.map viewProblem model.problems)
+                    , ul [ class "error-messages" ]
+                        (List.map viewProblem model.problems)
                     , viewForm model.form
                     ]
                 ]
             ]
         ]
+
+
+
+-- VALIDATION
+
+
+{-| to not send unvalidated fields
+-}
+type TrimmedForm
+    = Trimmed Form
+
+
+type ValidatedField
+    = Email
+    | Password
+    | Login
+    | Name
+
+
+fieldsToValidate : List ValidatedField
+fieldsToValidate =
+    [ Email
+    , Password
+    , Login
+    , Name
+    ]
+
+
+trimFields : Form -> TrimmedForm
+trimFields form =
+    Trimmed
+        { email = String.trim form.email
+        , password = String.trim form.password
+        , login = String.trim form.login
+        , name = String.trim form.name
+        }
+
+
+{-| Trim the form and validate its fields.
+-}
+validate : Form -> Result (List Problem) TrimmedForm
+validate form =
+    let
+        trimmedForm =
+            trimFields form
+    in
+    case List.concatMap (validateField trimmedForm) fieldsToValidate of
+        [] ->
+            Ok trimmedForm
+
+        problems ->
+            Err problems
+
+
+{-| Checks all the fields for errors.
+-}
+validateField : TrimmedForm -> ValidatedField -> List Problem
+validateField (Trimmed form) field =
+    let
+        emptyValidation str msg =
+            if String.isEmpty str then
+                [ msg ]
+
+            else
+                []
+
+        lengthValidation : String -> (Int -> Bool) -> String -> List String
+        lengthValidation str p msg =
+            if p (String.length str) then
+                []
+
+            else
+                [ msg ]
+
+        passwordValidation : String -> List String
+        passwordValidation pswd =
+            let
+                errEmpty =
+                    emptyValidation pswd "password can't be empty."
+
+                errLength =
+                    lengthValidation pswd (\x -> x > 8) "password must be > 8."
+            in
+            List.append errEmpty errLength
+    in
+    List.map (InvalidEntry field) <|
+        case field of
+            Email ->
+                emptyValidation form.email "email can't be blank."
+
+            Password ->
+                passwordValidation form.password
+
+            Login ->
+                emptyValidation form.login "login can't be blank."
+
+            Name ->
+                emptyValidation form.name "name can't be blank."
+
+
+viewProblem : Problem -> Html msg
+viewProblem problem =
+    let
+        errorMessage =
+            case problem of
+                InvalidEntry _ msg ->
+                    "вв"
+
+                ServerError str ->
+                    str
+    in
+    li [] [ text errorMessage ]