Expose a length property for curried functions (#477)

Co-authored-by: Dale Francis <[email protected]>

Author: JamieDixon

src/core/curry.js

@@ -43,6 +43,12 @@ function curry(fn) {
     value: true
   })
 
+  Object.defineProperty(curried, 'length', {
+    enumerable: false,
+    writable: false,
+    value: fn.length
+  })
+
   return curried
 }
 

src/core/curry.spec.js

@@ -78,3 +78,11 @@ test('curry on a curried function', t => {
 
   t.end()
 })
+
+test('curried function with arity exposed', t => {
+  const fn = (a, b) => a + b
+  const curried = curry(fn)
+  t.equal(curried.length, fn.length, 'returns the same arity as its uncurried counterpart')
+
+  t.end()
+})