Implement printer support for await expressions

Author: Maxim

src/res_parsetree_viewer.ml

@@ -65,6 +65,13 @@ let processFunctionAttributes attrs =
   in
   process false false [] attrs
 
+let hasAwaitAttribute attrs =
+  List.exists
+    (function
+      | {Location.txt = "await"}, _ -> true
+      | _ -> false)
+    attrs
+
 let collectListExpressions expr =
   let rec collect acc expr =
     match expr.pexp_desc with
@@ -178,8 +185,8 @@ let filterParsingAttrs attrs =
       match attr with
       | ( {
             Location.txt =
-              ( "ns.ternary" | "ns.braces" | "res.template" | "bs" | "ns.iflet"
-              | "ns.namedArgLoc" | "ns.optional" );
+              ( "ns.ternary" | "ns.braces" | "res.template" | "await" | "bs"
+              | "ns.iflet" | "ns.namedArgLoc" | "ns.optional" );
           },
           _ ) ->
         false

src/res_parsetree_viewer.mli

@@ -22,6 +22,8 @@ val processFunctionAttributes :
   Parsetree.attributes ->
   bool (* async *) * bool (* uncurried *) * Parsetree.attributes
 
+val hasAwaitAttribute : Parsetree.attributes -> bool
+
 type ifConditionKind =
   | If of Parsetree.expression
   | IfLet of Parsetree.pattern * Parsetree.expression

src/res_printer.ml

@@ -3278,6 +3278,11 @@ and printExpression ~customLayout (e : Parsetree.expression) cmtTbl =
     | Pexp_poly _ -> Doc.text "Pexp_poly not impemented in printer"
     | Pexp_object _ -> Doc.text "Pexp_object not impemented in printer"
   in
+  let exprWithAwait =
+    if ParsetreeViewer.hasAwaitAttribute e.pexp_attributes then
+      Doc.concat [Doc.text "await "; printedExpression]
+    else printedExpression
+  in
   let shouldPrintItsOwnAttributes =
     match e.pexp_desc with
     | Pexp_apply _ | Pexp_fun _ | Pexp_newtype _ | Pexp_setfield _
@@ -3289,12 +3294,11 @@ and printExpression ~customLayout (e : Parsetree.expression) cmtTbl =
     | _ -> false
   in
   match e.pexp_attributes with
-  | [] -> printedExpression
+  | [] -> exprWithAwait
   | attrs when not shouldPrintItsOwnAttributes ->
     Doc.group
-      (Doc.concat
-         [printAttributes ~customLayout attrs cmtTbl; printedExpression])
-  | _ -> printedExpression
+      (Doc.concat [printAttributes ~customLayout attrs cmtTbl; exprWithAwait])
+  | _ -> exprWithAwait
 
 and printPexpFun ~customLayout ~inCallback e cmtTbl =
   let attrsOnArrow, parameters, returnExpr = ParsetreeViewer.funExpr e in

tests/printer/expr/asyncAwait.res

@@ -10,4 +10,10 @@ let f = async () => ()
 let f = async (.) => ()
 let f = async f => f()
 let f = async (a, b) => a + b
-let f = async (. a, b) => a + b
+let f = async (. a, b) => a + b
+
+
+let maybeSomeValue = switch await fetchData(url) {    
+| data => Some(data)
+| exception JsError(_) => None
+}

tests/printer/expr/expected/asyncAwait.res.txt

@@ -1,8 +1,8 @@
 let sequentialAwait = async () => {
-  let result1 = @await paused("first")
+  let result1 = await paused("first")
   nodeJsAssert.equal(result1, "first")
 
-  let result2 = @await paused("second")
+  let result2 = await paused("second")
   nodeJsAssert.equal(result2, "second")
 }
 
@@ -11,3 +11,8 @@ let f = async (. ()) => ()
 let f = async (f) => f()
 let f = async (a, b) => a + b
 let f = async (. a, b) => a + b
+
+let maybeSomeValue = switch await fetchData(url) {
+| data => Some(data)
+| exception JsError(_) => None
+}