Base/JS: GHC.JS.Foreign.Callback module (issue 23126)wip/js-exports

* Add the Callback module for "exporting" Haskell functions
to be available to plain JavaScript code

* Fix some primitives defined in GHC.JS.Prim

* Add a JavaScript section to the user guide with instructions
on how to use the JavaScript FFI, building up to using Callbacks
to interact with the browser

* Add tests for the JavaScript FFI and Callbacks

1 files changed, 33 insertions, 0 deletions

diff --git a/testsuite/tests/javascript/js-callback03.hs b/testsuite/tests/javascript/js-callback03.hs
new file mode 100644
index 0000000000..311c2634f4
--- /dev/null
+++ b/testsuite/tests/javascript/js-callback03.hs
@@ -0,0 +1,33 @@
+import GHC.JS.Prim
+import GHC.JS.Foreign.Callback
+
+foreign import javascript "((f) => { globalF = f; })"
+  setF :: Callback (JSVal -> IO ()) -> IO ()
+
+foreign import javascript "((x) => { globalF(x); })"
+  callF :: JSVal -> IO ()
+
+foreign import javascript "((x,y) => { return x + y })"
+  js_plus :: JSVal -> JSVal -> IO JSVal
+
+foreign import javascript "((g) => { globalG = g; })"
+  setG :: Callback (JSVal -> JSVal -> IO JSVal) -> IO ()
+
+foreign import javascript "((x,y) => { return globalG(x,y); })"
+  callG :: JSVal -> JSVal -> IO JSVal
+
+main :: IO ()
+main = do
+  -- Set functions globally on the JavaScript side, to be accessed in regular JavaScript code
+  f <- syncCallback1 ThrowWouldBlock (\x -> if isNull x then putStrLn "isNull" else putStrLn "isNotNull")
+  g <- syncCallback2' js_plus
+  setF f
+  setG g
+
+  -- Do other things before using the globally-set functions
+  putStrLn "test"
+
+  -- Use the globally-set functions
+  callF jsNull
+  callF $ toJSString ""
+  print . fromJSInt =<< callG (toJSInt 1) (toJSInt 2)