Uncurried Mode docs

Author: fhammerschmidt

data/sidebar_manual_latest.json

@@ -32,6 +32,7 @@
     "import-export",
     "attribute",
     "unboxed",
+    "uncurried-mode",
     "reserved-keywords"
   ],
   "Advanced Features": [

pages/docs/manual/latest/uncurried-mode.mdx

@@ -0,0 +1,131 @@
+---
+title: "Uncurried Mode"
+description: "Uncurried Mode"
+canonical: "/docs/manual/latest/uncurried-mode"
+---
+
+# Uncurried Mode
+
+Since ReScript 11, the language compiles in "uncurried" mode by default.
+Before that, currying in ReScript looked like this:
+
+<CodeTab labels={["ReScript", "JS Output"]}>
+
+```res
+let add = (a, b) => a + b
+let addFive = add(5)
+```
+
+```js
+function add(a, b) {
+  return a + b | 0;
+}
+
+function addFive(param) {
+  return 5 + param | 0;
+}
+```
+
+</CodeTab>
+
+It is shorter than having to write all remaining parameters again.
+
+<CodeTab labels={["ReScript", "JS Output"]}>
+
+```res
+let add = (a, b) => a + b
+let addFive = (b) => add(5, b)
+```
+
+```js
+function add(a, b) {
+  return a + b | 0;
+}
+
+function addFive(b) {
+  return 5 + b | 0;
+}
+```
+
+</CodeTab>
+
+In ReScript 11, that would yield an error.
+
+```rescript
+let add = (a, b) => a + b
+let addFive = add(5) // <-- Error:
+// This uncurried function has type (. int, int) => int
+// It is applied with 1 arguments but it requires 2.
+```
+
+To fix it, you can just state the remaining parameters explicitly.
+
+<CodeTab labels={["ReScript", "JS Output"]}>
+```res
+let add = (a, b) => a + b
+let addFive = (b) => add(5, b)
+```
+
+```js
+function add(a, b) {
+  return a + b | 0;
+}
+
+function addFive(b) {
+  return 5 + b | 0;
+}
+```
+</CodeTab>
+
+Or use the new explicit syntax for partial application.
+
+<CodeTab labels={["ReScript", "JS Output"]}>
+```res
+let add = (a, b) => a + b
+let addFive = add(5, ...)
+```
+
+```js
+function add(a, b) {
+  return a + b | 0;
+}
+
+function addFive(extra) {
+  return 5 + extra | 0;
+}
+```
+</CodeTab>
+
+The former approach helps library authors support both ReScript 11 and earlier versions.
+
+### No final unit necessary
+
+In Uncurried Mode, the "final unit" pattern is not necessary anymore, while optional or default parameters are still supported.
+
+```res
+// old 
+let myFun = (~name=?, ())
+
+// new
+let myFun = (~name=?)
+```
+
+### How to switch back to curried mode
+
+While we strongly encourage all users to switch to the new uncurried mode, it is still possible to opt out. Just add a
+
+```json
+{
+  "uncurried": false
+}
+```
+
+to your `rescript.json`, and your project will be compiled in curried mode again.
+
+If you have uncurried mode off and still want to try it on a per-file basis, you can turn it on via 
+
+```rescript
+@@uncurried
+```
+
+at the top of a `.res` file.