Allow source file to be passed to repl (#396)

README.rst

@@ -30,6 +30,13 @@ To try out *Source* in a REPL, run
 
   $ node dist/repl/repl.js [chapter] # default: 1
 
+If you wish, you can pass in a file path instead, to evaluate some *Source* before initialising the REPL
+It will be run in *Source* chapter 4.
+
+.. code-block::
+
+  $ node dist/repl/repl.js [path/to/file]
+
 or alternatively, install js-slang and run
 
 .. code-block::

src/repl/repl.ts

@@ -1,29 +1,48 @@
+import fs = require('fs')
 import repl = require('repl') // 'repl' here refers to the module named 'repl' in index.d.ts
-import { createContext, parseError, runInContext } from '../index'
+import { createContext, IOptions, parseError, runInContext } from '../index'
 
-function startRepl(chapter = 1, useSubst: boolean) {
+function startRepl(chapter = 1, useSubst: boolean, prelude = '') {
   // use defaults for everything
   const context = createContext(chapter)
-  repl.start(
-    // the object being passed as argument fits the interface ReplOptions in the repl module.
-    {
-      eval: (cmd, unusedContext, unusedFilename, callback) => {
-        runInContext(cmd, context, { scheduler: 'preemptive', useSubst }).then(obj => {
-          if (obj.status === 'finished') {
-            callback(null, obj.value)
-          } else {
-            callback(new Error(parseError(context.errors)), undefined)
+  const options: Partial<IOptions> = { scheduler: 'preemptive', useSubst }
+  runInContext(prelude, context, options).then(preludeResult => {
+    if (preludeResult.status === 'finished') {
+      console.log(preludeResult.value)
+      repl.start(
+        // the object being passed as argument fits the interface ReplOptions in the repl module.
+        {
+          eval: (cmd, unusedContext, unusedFilename, callback) => {
+            runInContext(cmd, context, options).then(obj => {
+              if (obj.status === 'finished') {
+                callback(null, obj.value)
+              } else {
+                callback(new Error(parseError(context.errors)), undefined)
+              }
+            })
           }
-        })
-      }
+        }
+      )
+    } else {
+      throw new Error(parseError(context.errors))
     }
-  )
+  })
 }
 
 function main() {
-  const chapter = process.argv.length > 2 ? parseInt(process.argv[2], 10) : 1
-  const useSubst = process.argv.length > 3 ? process.argv[3] === 'subst' : false
-  startRepl(chapter, useSubst)
+  const firstArg = process.argv[2]
+  if (process.argv.length === 3 && String(Number(firstArg)) !== firstArg.trim()) {
+    fs.readFile(firstArg, 'utf8', (err, data) => {
+      if (err) {
+        throw err
+      }
+      startRepl(4, false, data)
+    })
+  } else {
+    const chapter = process.argv.length > 2 ? parseInt(firstArg, 10) : 1
+    const useSubst = process.argv.length > 3 ? process.argv[3] === 'subst' : false
+    startRepl(chapter, useSubst)
+  }
 }
 
 main()