Expand sourceType explanation

Author: GeoffreyBooth

lib/coffeescript/nodes.js

@@ -796,9 +796,18 @@ exports.Block = class Block extends Base
 
   astProperties: ->
     return
-      # For now, we are not including `sourceType` on the `Program` AST node,
-      # as we'd like to have a reliable way to determine its proper value and
-      # things like the ongoing Node modules discussion make that unclear.
+      # For now, we’re not including `sourceType` on the `Program` AST node.
+      # Its value could be either `'script'` or `'module'`, and there’s no way
+      # for CoffeeScript to always know which it should be. The presence of an
+      # `import` or `export` statement in source code would imply that it should
+      # be a `module`, but a project may consist of mostly such files and also
+      # an outlier file that lacks `import` or `export` but is still imported
+      # into the project and therefore expects to be treated as a `module`.
+      # Determining the value of `sourceType` is essentially the same challenge
+      # posed by determining the parse goal of a JavaScript file, also `module`
+      # or `script`, and so if Node figures out a way to do so for `.js` files
+      # then CoffeeScript can copy Node’s algorithm.
+
       # sourceType: 'module'
       body: @bodyToAst()
       directives: []