Merge pull request #24 from justinwoo/builder-modify

add Builder.modify

Author: hdgarrood

src/Data/Record/Builder.js

@@ -19,6 +19,15 @@ exports.unsafeInsert = function(l) {
   };
 };
 
+exports.unsafeModify = function(l) {
+  return function (f) {
+    return function(rec) {
+      rec[l] = f(rec[l]);
+      return rec;
+    };
+  };
+};
+
 exports.unsafeDelete = function(l) {
   return function(rec) {
     delete rec[l];

src/Data/Record/Builder.purs

@@ -2,6 +2,7 @@ module Data.Record.Builder
   ( Builder
   , build
   , insert
+  , modify
   , delete
   , rename
   , merge
@@ -14,6 +15,7 @@ import Type.Row (class RowLacks)
 
 foreign import copyRecord :: forall r1. Record r1 -> Record r1
 foreign import unsafeInsert :: forall a r1 r2. String -> a -> Record r1 -> Record r2
+foreign import unsafeModify :: forall a b r1 r2. String -> (a -> b) -> Record r1 -> Record r2
 foreign import unsafeDelete :: forall r1 r2. String -> Record r1 -> Record r2
 foreign import unsafeRename :: forall r1 r2. String -> String -> Record r1 -> Record r2
 foreign import unsafeMerge :: forall r1 r2 r3. Record r1 -> Record r2 -> Record r3
@@ -49,6 +51,17 @@ insert
   -> Builder (Record r1) (Record r2)
 insert l a = Builder \r1 -> unsafeInsert (reflectSymbol l) a r1
 
+-- | Build by modifying an existing field.
+modify
+  :: forall l a b r r1 r2
+   . RowCons l a r r1
+  => RowCons l b r r2
+  => IsSymbol l
+  => SProxy l
+  -> (a -> b)
+  -> Builder (Record r1) (Record r2)
+modify l f = Builder \r1 -> unsafeModify (reflectSymbol l) f r1
+
 -- | Build by deleting an existing field.
 delete
   :: forall l a r1 r2

test/Main.purs

@@ -49,7 +49,8 @@ main = do
   let testBuilder = Builder.build (Builder.insert x 42
                                   >>> Builder.merge { y: true, z: "testing" }
                                   >>> Builder.delete y
+                                  >>> Builder.modify x show
                                   >>> Builder.rename z y) {}
 
   assert' "Data.Record.Builder" $
-    testBuilder.x == 42 && testBuilder.y == "testing"
+    testBuilder.x == "42" && testBuilder.y == "testing"