From 4c4717f9bdbc1c3c69bb85348129185c20aba4e6 Mon Sep 17 00:00:00 2001
From: ilias <mail@ilias.xyz>
Date: Wed, 7 Feb 2018 14:32:21 +0100
Subject: [PATCH] Allow reusing validators

Some sort of contravariant map would allow reusing validators in different
contexts.

Alternative names proposed in Slack by norpan and gyzerok:

- `comap`
- `contraMap`
- `select` (i.e. `Validate.select .name nameValidator`)
---
 src/Validate.elm | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/src/Validate.elm b/src/Validate.elm
index 211abfa..c48ac70 100644
--- a/src/Validate.elm
+++ b/src/Validate.elm
@@ -16,6 +16,7 @@ module Validate
         , isBlank
         , isInt
         , isValidEmail
+        , preMap
         , validate
         )
 
@@ -57,6 +58,11 @@ module Validate
 @docs all, any, firstError
 
 
+# Reusing validators
+
+@docs preMap
+
+
 # Checking values directly
 
 @docs isBlank, isInt, isValidEmail
@@ -220,6 +226,33 @@ ifFalse test error =
 
 
 
+-- REUSING VALIDATORS --
+
+
+{-| Reuse a validator in a larger context.
+
+    import Validate exposing (ifBlank, preMap)
+
+    type alias User =
+        { name : String
+        }
+
+    nameValidator : Validator String String
+    nameValidator =
+        ifBlank identity
+            "Please enter a name"
+
+    userValidator : Validator String User
+    userValidator =
+        preMap .user nameValidator
+
+-}
+preMap : (large -> small) -> Validator error small -> Validator error large
+preMap f (Validator validator) =
+    Validator (f >> validator)
+
+
+
 -- COMBINING VALIDATORS --