fable-core.js generation from TypeScript (#240)

* [WIP: DO NOT MERGE] Adds es2015-mini.ts (es2015.js ported to TypeScript). Initial proof of concept: List and (partial) Seq class only.

* Fix build target 'FableCore' on Windows.

* Adds TypeScript compilation in build target FableCore.

- Renames es2015-mini.ts to fable-core.tsc.ts

* Adds Symbol, Choice, Util, TimeSpan, Date and Timer (not strongly typed) to fable-core.tsc.ts.

* Add remaining classes to fable-core.tsc.ts

* (!) Builds fable-core.js from TypeScript sources. -- Builds are ok. Tests are not.

* Latest fixes to fable-core.tsc.ts. All tests passing now.

* Expands some test cases.

* Adds tests removed from commit ("Expands some test cases." / db0a0e5)

build.fsx

@@ -264,16 +264,27 @@ Target "FableCore" (fun _ ->
         | true -> true) false
     |> ignore
 
+    // Builds fable-core.babel.js from ES2015 sources.
     try
-        sprintf "es2015.js -o fable-core.js --plugins %s-umd" babelPlugin |> run "babel"
+        sprintf "es2015.js -o fable-core.babel.js --plugins %s-umd --no-babelrc" babelPlugin |> run "babel"
         // The commonjs version is for Fuse, which is not compatible with UMD
-        sprintf "es2015.js -o commonjs.js --plugins %s-commonjs" babelPlugin |> run "babel"
+        sprintf "es2015.js -o commonjs.babel.js --plugins %s-commonjs --no-babelrc" babelPlugin |> run "babel"
     with
-    | _ -> failwith "Cannot find Babel CLI, please run `npm i -g babel-cli`"
+    | _ -> failwith "Cannot find Babel CLI. Please run `npm i -g babel-cli`"
+
+    // Builds fable-core.js from TypeScript sources.
+    //   FIXME: Requires 'npm install babel-preset-es2015' in src/fable/Fable.Core/npm
+    try "fable-core.tsc.ts --target ES2015 --declaration" |> run "tsc"
+    with _ -> failwith "Cannot find TypeScript compiler. Please run `npm i -g typescript`"
+    setEnvironVar "BABEL_ENV" "target-umd"
+    "fable-core.tsc.js -o fable-core.js" |> run "babel"
+    setEnvironVar "BABEL_ENV" "target-commonjs"
+    "fable-core.tsc.js -o commonjs.js" |> run "babel"
+    fableCoreNpmDir + "/fable-core.tsc.js" |> FileUtils.rm
 
     if environVar "DEV_MACHINE" = "1" then
-        try "fable-core.js -c -m -o fable-core.min.js" |> run "uglifyjs"
-        with _ -> failwith "Cannot find uglify-js, please run `npm i -g uglify-js`"
+        try "fable-core.babel.js -c -m -o fable-core.min.js" |> run "uglifyjs"
+        with _ -> failwith "Cannot find uglify-js. Please run `npm i -g uglify-js`"
 
     // Update Fable.Core version
     Util.assemblyInfo "src/fable/Fable.Core/" fableCoreVersion []

src/fable/Fable.Compiler/Replacements.fs

@@ -893,7 +893,10 @@ module private AstPass =
         | "head" | "tail" | "length" | "count" ->
             let meth = if meth = "count" then "length" else meth
             match kind with
-            | Seq -> ccall "Seq" meth (staticArgs c args)
+            | Seq -> 
+                // A static 'length' method causes problems in JavaScript -- https://github.com/Microsoft/TypeScript/issues/442
+                let seqMeth = if meth = "length" then "_length" else meth
+                ccall "Seq" seqMeth (staticArgs c args)
             | List -> let c, _ = instanceArgs c args in prop meth c
             | Array ->
                 let c, _ = instanceArgs c args

src/fable/Fable.Core/npm/.babelrc

@@ -0,0 +1,32 @@
+{
+  "env": {
+    "target-umd": {
+      "presets": [
+        "es2015"
+      ],
+      "plugins": [
+        "transform-es2015-modules-umd",
+        [
+          "transform-es2015-classes",
+          {
+            "loose": true
+          }
+        ]
+      ]
+    },
+    "target-commonjs": {
+      "presets": [
+        "es2015"
+      ],
+      "plugins": [
+        "transform-es2015-modules-commonjs",
+        [
+          "transform-es2015-classes",
+          {
+            "loose": true
+          }
+        ]
+      ]
+    }
+  }
+}