Enable default R location to be used on mac/linux if none is supplied

src/util.ts

@@ -2,16 +2,40 @@
 
 import { existsSync } from 'fs-extra';
 import path = require('path');
+import fs = require('fs');
 import { window, workspace } from 'vscode';
 import winreg = require('winreg');
 
 export function config() {
     return workspace.getConfiguration('r');
 }
 
+function getRfromEnvPath(platform: string) {
+    let splitChar: string = ':';
+    let fileExtension: string = '';
+
+    if (platform === 'win32') {
+        splitChar = ';';
+        fileExtension = '.exe';
+    }
+
+    const os_paths: string[]|string = process.env.PATH.split(splitChar);
+    for (const os_path of os_paths) {
+        const os_r_path: string = path.join(os_path, 'R' + fileExtension);
+        if (fs.existsSync(os_r_path)) {
+            return os_r_path;
+        }
+    }
+    return '';
+}
+
 export async function getRpath() {
-    if (process.platform === 'win32') {
-        let rpath: string = config().get<string>('rterm.windows');
+
+    let rpath: string = '';
+    const platform: string = process.platform;
+
+    if ( platform === 'win32') {
+        rpath = config().get<string>('rterm.windows');
         if (rpath === '') {
             // Find path from registry
             try {
@@ -26,17 +50,19 @@ export async function getRpath() {
                 rpath = '';
             }
         }
-
-        return rpath;
+    } else if (platform === 'darwin') {
+        rpath = config().get<string>('rterm.mac');
+    } else if (platform === 'linux') {
+        rpath = config().get<string>('rterm.linux');
     }
-    if (process.platform === 'darwin') {
-        return config().get<string>('rterm.mac');
+
+    if (rpath === '') {
+        rpath = getRfromEnvPath(platform);
     }
-    if (process.platform === 'linux') {
-        return config().get<string>('rterm.linux');
+    if (rpath !== '') {
+        return rpath;
     }
     window.showErrorMessage(`${process.platform} can't use R`);
-
     return undefined;
 }