diff --git a/README.md b/README.md
index 9b51cf6..396107b 100644
--- a/README.md
+++ b/README.md
@@ -11,6 +11,7 @@ options are available for the user to configure, in `settings.json`:
 {
   elixir.formatter: {
     mixFormatArgs: "--dry-run",
+    mixCommandPath: "/etc/custom/elixir/path/bin/mix",
     formatterCwd: "../some/dir/to/run/mix/format/from"
   }
 }
diff --git a/src/extension.ts b/src/extension.ts
index bd04cd9..468bdeb 100644
--- a/src/extension.ts
+++ b/src/extension.ts
@@ -1,49 +1,99 @@
-import { languages, ExtensionContext } from "vscode";
-import { workspace, Range, TextDocument, TextEdit, window } from "vscode";
-import cp = require("child_process");
-var path = require('path')
+import * as path from "path";
+
+import { ExtensionContext, languages, window } from "vscode";
+import { Range, TextDocument, TextEdit, workspace } from "vscode";
+
+import { spawn } from "child_process";
 
 function fullDocumentRange(document: TextDocument): Range {
   const lastLineId = document.lineCount - 1;
   return new Range(0, 0, lastLineId, document.lineAt(lastLineId).text.length);
 }
 
-function format(document: TextDocument): Promise<TextEdit[]> {
+function helpfulMixErrorMessage(error: any): string {
+  if (error.code === "ENOENT") {
+    return (
+      (error.path || "mix") +
+      " command not found. It was expected to be in $PATH: " +
+      process.env.PATH +
+      ". Note that VSCode is running in an environment different from your terminal, " +
+      "and it doesn't read your shell rc files. You can set elixir.formatter.mixCommandPath " +
+      "to a custom location if it's not in your $PATH."
+    );
+  }
+  // Get rid of standard header to leave space in the error popup
+  // for the actual line number
+  return error.message.replace("mix format failed for stdin\n", "");
+}
+
+function format(document: TextDocument): Promise<string> {
   return new Promise((resolve, reject) => {
     // Create mix command
-    const mixFormatArgs: string = workspace.getConfiguration("elixir.formatter").get("mixFormatArgs") || "";
-    const cmd = `mix format ${mixFormatArgs} ${document.fileName}`;
+    const mixCommandPath: string =
+      workspace.getConfiguration("elixir.formatter").get("mixCommandPath") ||
+      "/bin/mix";
+    const mixFormatArgsSetting: string = workspace
+      .getConfiguration("elixir.formatter")
+      .get("mixFormatArgs");
+    const mixFormatArgs =
+      typeof mixFormatArgsSetting === "string" && mixFormatArgsSetting !== ""
+        ? mixFormatArgsSetting.split(" ")
+        : [];
 
     // Figure out the working directory to run mix format in
     const workspaceRootPath = workspace.rootPath ? workspace.rootPath : "";
-    const relativePath: string = workspace.getConfiguration("elixir.formatter").get("formatterCwd") || "";
+    const relativePath: string =
+      workspace.getConfiguration("elixir.formatter").get("formatterCwd") || "";
     const cwd = path.resolve(workspaceRootPath, relativePath);
 
-    // Run the command
-    cp.exec(
-      cmd,
-      {
-        cwd
-      },
-      function(error, stdout, stderr) {
-        if (error !== null) {
-          const message = `Cannot format due to syntax errors.: ${stderr}`;
-          window.showErrorMessage(message);
-          return reject(message);
-        } else {
-          return [TextEdit.replace(fullDocumentRange(document), stdout)];
-        }
+    const proc = spawn(mixCommandPath, ["format", ...mixFormatArgs, "-"], {
+      cwd
+    });
+    proc.on("error", reject);
+
+    // If process fails to start, write syscall will fail synchronously and
+    // will mask the original error message. Let's postpone writing until
+    // all event handlers are setup and NodeJS had a chance to call the
+    // on("error") callback.
+    process.nextTick(() => {
+      proc.stdin.write(document.getText(), "utf8", error =>
+        error ? "reject(error)" : proc.stdin.end()
+      );
+    });
+
+    const stdout = [];
+    const stderr = [];
+    proc.stdout.setEncoding("utf8");
+    proc.stderr.setEncoding("utf8");
+    proc.stdout.on("data", data => stdout.push(data));
+    proc.stderr.on("data", data => stderr.push(data));
+
+    proc.on("exit", code => {
+      if (code === 0) {
+        resolve(stdout.join(""));
+      } else {
+        const error: any = new Error(stderr.join(""));
+        error.code = code;
+        reject(error);
       }
-    );
+    });
   });
 }
 
 export function activate(context: ExtensionContext) {
-  languages.registerDocumentFormattingEditProvider('elixir', {
-    provideDocumentFormattingEdits(document: TextDocument): Thenable<TextEdit[]> {
-      return document.save().then(() => {
-        return format(document);
-      });
+  languages.registerDocumentFormattingEditProvider("elixir", {
+    provideDocumentFormattingEdits(
+      document: TextDocument
+    ): Thenable<TextEdit[]> {
+      return format(document).then(
+        formatted => {
+          return [TextEdit.replace(fullDocumentRange(document), formatted)];
+        },
+        error => {
+          window.showErrorMessage(helpfulMixErrorMessage(error));
+          throw error;
+        }
+      );
     }
   });
-}
\ No newline at end of file
+}