From 3326f477dc74752bf1ef0eb64d173b4b5e88e748 Mon Sep 17 00:00:00 2001
From: Matthew Leon <ml@matthewleon.com>
Date: Sat, 10 Feb 2018 20:13:33 -0800
Subject: [PATCH] ST.modify

addresses https://github.com/purescript/purescript-record/issues/31
---
 src/Data/Record/ST.js   | 10 ++++++++++
 src/Data/Record/ST.purs | 21 +++++++++++++++++++++
 test/Main.purs          |  6 ++++++
 3 files changed, 37 insertions(+)

diff --git a/src/Data/Record/ST.js b/src/Data/Record/ST.js
index 12b97c4..9c3bf8a 100644
--- a/src/Data/Record/ST.js
+++ b/src/Data/Record/ST.js
@@ -45,3 +45,13 @@ exports.unsafePokeSTRecord = function(l) {
     };
   };
 };
+
+exports.unsafeModify = function(l) {
+  return function(f) {
+    return function(rec) {
+      return function() {
+        rec[l] = f(rec[l]);
+      };
+    };
+  };
+};
diff --git a/src/Data/Record/ST.purs b/src/Data/Record/ST.purs
index 671752f..f850c27 100644
--- a/src/Data/Record/ST.purs
+++ b/src/Data/Record/ST.purs
@@ -4,6 +4,7 @@ module Data.Record.ST
   , thawSTRecord
   , peekSTRecord
   , pokeSTRecord
+  , modify
   , runSTRecord
   , pureSTRecord
   ) where
@@ -69,3 +70,23 @@ pokeSTRecord
   -> STRecord h r
   -> Eff (st :: ST h | eff) Unit
 pokeSTRecord l = unsafePokeSTRecord (reflectSymbol l)
+
+foreign import unsafeModify
+  :: forall a r h eff
+   . String
+  -> (a -> a)
+  -> STRecord h r
+  -> Eff (st :: ST h | eff) Unit
+
+-- | Modify a record in place,
+-- | by providing a type-level representative for the label to update
+-- | and a function to update it.
+modify
+  :: forall l h a r r1 eff
+   . RowCons l a r1 r
+  => IsSymbol l
+  => SProxy l
+  -> (a -> a)
+  -> STRecord h r
+  -> Eff (st :: ST h | eff) Unit
+modify l = unsafeModify (reflectSymbol l)
diff --git a/test/Main.purs b/test/Main.purs
index 1c2a34b..263436b 100644
--- a/test/Main.purs
+++ b/test/Main.purs
@@ -6,6 +6,7 @@ import Control.Monad.Eff (Eff)
 import Data.Record (delete, equal, get, insert, modify, rename, set)
 import Data.Record.Builder as Builder
 import Data.Record.ST (pokeSTRecord, pureSTRecord, thawSTRecord)
+import Data.Record.ST as ST
 import Data.Record.Unsafe (unsafeHas)
 import Data.Symbol (SProxy(..))
 import Test.Assert (ASSERT, assert')
@@ -42,9 +43,14 @@ main = do
         pokeSTRecord x 42 rec
         pokeSTRecord y "testing" rec
         pure rec
+      stTest2 = pureSTRecord do
+        rec <- thawSTRecord { x: 41 }
+        ST.modify x (_ + 1) rec
+        pure rec
 
   assert' "pokeSTRecord" $
     stTest1.x == 42 && stTest1.y == "testing"
+  assert' "ST.modify" $ stTest2.x == 42
 
   let testBuilder = Builder.build (Builder.insert x 42
                                   >>> Builder.merge { y: true, z: "testing" }