Remove array allocation and local array ffi from the Show instance for records (#299)

* Don't allocate an array to show a record

* Update changelog

* Simplify the code

Author: ajnsit

CHANGELOG.md

@@ -12,6 +12,7 @@ Bugfixes:
 
 Other improvements:
 - Documentation: Clarify relationship between `Ord` and `Eq` (#298 by @JamieBallingall)
+- Remove array allocation and local array FFI from the `Show` instance for records. (#299 by @ajnsit)
 
 ## [v6.0.0](https://github.com/purescript/purescript-prelude/releases/tag/v6.0.0) - 2022-04-27
 

src/Data/Show.js

@@ -57,15 +57,3 @@ export const showArrayImpl = function (f) {
     return "[" + ss.join(",") + "]";
   };
 };
-
-export const cons = function (head) {
-  return function (tail) {
-    return [head].concat(tail);
-  };
-};
-
-export const intercalate = function (separator) {
-  return function (xs) {
-    return xs.join(separator);
-  };
-};

src/Data/Show.purs

@@ -5,7 +5,10 @@ module Data.Show
   , showRecordFields
   ) where
 
+import Data.Semigroup ((<>))
 import Data.Symbol (class IsSymbol, reflectSymbol)
+import Data.Unit (Unit)
+import Data.Void (Void, absurd)
 import Prim.Row (class Nub)
 import Prim.RowList as RL
 import Record.Unsafe (unsafeGet)
@@ -20,6 +23,9 @@ import Type.Proxy (Proxy(..))
 class Show a where
   show :: a -> String
 
+instance showUnit :: Show Unit where
+  show _ = "unit"
+
 instance showBoolean :: Show Boolean where
   show true = "true"
   show false = "false"
@@ -42,32 +48,43 @@ instance showArray :: Show a => Show (Array a) where
 instance showProxy :: Show (Proxy a) where
   show _ = "Proxy"
 
+instance showVoid :: Show Void where
+  show = absurd
+
 instance showRecord ::
   ( Nub rs rs
   , RL.RowToList rs ls
   , ShowRecordFields ls rs
   ) =>
   Show (Record rs) where
-  show record = case showRecordFields (Proxy :: Proxy ls) record of
-    [] -> "{}"
-    fields -> intercalate " " [ "{", intercalate ", " fields, "}" ]
+  show record = "{" <> showRecordFields (Proxy :: Proxy ls) record <> "}"
 
 -- | A class for records where all fields have `Show` instances, used to
 -- | implement the `Show` instance for records.
 class ShowRecordFields :: RL.RowList Type -> Row Type -> Constraint
 class ShowRecordFields rowlist row where
-  showRecordFields :: Proxy rowlist -> Record row -> Array String
+  showRecordFields :: Proxy rowlist -> Record row -> String
 
 instance showRecordFieldsNil :: ShowRecordFields RL.Nil row where
-  showRecordFields _ _ = []
-
+  showRecordFields _ _ = ""
+else
+instance showRecordFieldsConsNil ::
+  ( IsSymbol key
+  , Show focus
+  ) =>
+  ShowRecordFields (RL.Cons key focus RL.Nil) row where
+  showRecordFields _ record = " " <> key <> ": " <> show focus <> " "
+    where
+    key = reflectSymbol (Proxy :: Proxy key)
+    focus = unsafeGet key record :: focus
+else
 instance showRecordFieldsCons ::
   ( IsSymbol key
   , ShowRecordFields rowlistTail row
   , Show focus
   ) =>
   ShowRecordFields (RL.Cons key focus rowlistTail) row where
-  showRecordFields _ record = cons (intercalate ": " [ key, show focus ]) tail
+  showRecordFields _ record = " " <> key <> ": " <> show focus <> "," <> tail
     where
     key = reflectSymbol (Proxy :: Proxy key)
     focus = unsafeGet key record :: focus
@@ -78,5 +95,3 @@ foreign import showNumberImpl :: Number -> String
 foreign import showCharImpl :: Char -> String
 foreign import showStringImpl :: String -> String
 foreign import showArrayImpl :: forall a. (a -> String) -> Array a -> String
-foreign import cons :: forall a. a -> Array a -> Array a
-foreign import intercalate :: String -> Array String -> String

src/Data/Unit.purs

@@ -1,7 +1,5 @@
 module Data.Unit where
 
-import Data.Show (class Show)
-
 -- | The `Unit` type has a single inhabitant, called `unit`. It represents
 -- | values with no computational content.
 -- |
@@ -14,6 +12,3 @@ foreign import data Unit :: Type
 
 -- | `unit` is the sole inhabitant of the `Unit` type.
 foreign import unit :: Unit
-
-instance showUnit :: Show Unit where
-  show _ = "unit"

src/Data/Void.purs

@@ -1,7 +1,5 @@
 module Data.Void (Void, absurd) where
 
-import Data.Show (class Show)
-
 -- | An uninhabited data type. In other words, one can never create
 -- | a runtime value of type `Void` because no such value exists.
 -- |
@@ -21,9 +19,6 @@ import Data.Show (class Show)
 -- | the `void` of C-family languages above.
 newtype Void = Void Void
 
-instance showVoid :: Show Void where
-  show = absurd
-
 -- | Eliminator for the `Void` type.
 -- | Useful for stating that some code branch is impossible because you've
 -- | "acquired" a value of type `Void` (which you can't).

test/Test/Main.purs

@@ -124,7 +124,9 @@ testRecordInstances :: AlmostEff
 testRecordInstances = do
   assert "Record equality" $ { a: 1 } == { a: 1 }
   assert "Record inequality" $ { a: 2 } /= { a: 1 }
-  assert "Record show" $ show { a: 1 } == "{ a: 1 }"
+  assert "Record show nil" $ show { } == "{}"
+  assert "Record show one" $ show { a: 1 } == "{ a: 1 }"
+  assert "Record show more" $ show { a: 1, b: 2 } == "{ a: 1, b: 2 }"
   assert "Record +" $ ({ a: 1, b: 2.0 } + { a: 0, b: (-2.0) }) == { a: 1, b: 0.0 }
   assert "Record *" $ ({ a: 1, b: 2.0 } * { a: 0, b: (-2.0) }) == { a: 0, b: -4.0 }
   assert "Record one" $ one == { a: 1, b: 1.0 }