Understanding Emacs Lisp Progn Vs Sequential Evaluation Of Scroll-up And Goto-char

In the realm of Emacs Lisp, the subtle nuances of expression evaluation can significantly impact the behavior of your code. This article delves into a fascinating observation regarding the difference between using progn to group expressions and evaluating them sequentially, specifically in the context of scrolling and point movement within a buffer. We will explore how (progn (scroll-up 1) (goto-char 0)) behaves differently from evaluating (scroll-up 1) followed by (goto-char 0), and why this distinction matters for Emacs Lisp developers.

At first glance, one might assume that grouping expressions with progn yields the same result as evaluating them one after another. However, Emacs Lisp, like many Lisp dialects, possesses unique characteristics that necessitate a closer examination. Let's consider the specific scenario presented: the interaction between scroll-up and goto-char.

When you evaluate (scroll-up 1) followed by (goto-char 0), the expected outcome occurs: the window scrolls up by one line, and then the point (cursor) moves to the beginning of the buffer. This sequential execution aligns with our intuitive understanding of how these functions should operate. However, when we encapsulate these expressions within a progn, as in (progn (scroll-up 1) (goto-char 0)), the behavior changes subtly yet significantly.

With progn, the window still scrolls up, but the point movement to the beginning of the buffer might not be as consistently observed. This discrepancy arises from the way Emacs handles window updates and redisplay. The scroll-up function, by its nature, triggers a redisplay of the window to reflect the new scroll position. When goto-char is evaluated immediately after scroll-up within a progn, the redisplay triggered by scroll-up might not have fully completed before goto-char is executed. This race condition can lead to the point moving to the beginning of the original buffer content, before the scroll operation was visually reflected. Understanding this behavior is crucial for writing robust Emacs Lisp code that interacts with the display.

To fully grasp the discrepancy, let's dissect the individual functions involved:

scroll-up

The scroll-up function, as its name suggests, scrolls the current window upwards by a specified number of lines. Its primary purpose is to bring content that was previously below the visible portion of the buffer into view. Crucially, scroll-up also triggers a redisplay of the window, signaling to Emacs that the visual representation needs to be updated to reflect the new scroll position. This redisplay operation, while essential for maintaining an accurate display, introduces a degree of asynchronicity into the process. Emacs attempts to optimize display updates to maintain responsiveness, which means that the redisplay triggered by scroll-up might not happen instantaneously. The scroll-up function inherently interacts with the window system, which is an external component to the core Emacs Lisp runtime. This interaction introduces a degree of latency, making the redisplay operation a non-atomic event.

When writing Emacs Lisp code that manipulates the display, it's important to be aware of this asynchronicity. Blindly assuming that the display has been updated immediately after calling a function like scroll-up can lead to unexpected behavior, especially when subsequent operations depend on the updated display state. This is precisely what we observe in the progn example, where goto-char might execute before the window has visually scrolled.

goto-char

The goto-char function is a fundamental Emacs Lisp primitive for moving the point (cursor) to a specific character position within the buffer. It takes a single argument, the character position, and directly modifies the point's location. Unlike scroll-up, goto-char does not inherently trigger a redisplay. Its effect on the display is indirect; the point's movement will become visible as part of the normal redisplay cycle, which Emacs manages to maintain an up-to-date view of the buffer.

The behavior of goto-char is generally predictable and synchronous. When you call goto-char, the point is moved to the specified position, and subsequent operations will operate on the buffer content at that new location. However, its interaction with asynchronous operations like scroll-up can introduce subtle complexities. If goto-char is executed before a redisplay triggered by scroll-up has completed, it might move the point relative to the old display state, leading to the observed discrepancy.

The progn special form in Lisp serves as a mechanism for grouping multiple expressions into a single unit. It evaluates each expression in the group sequentially and returns the value of the last expression. In the context of our example, (progn (scroll-up 1) (goto-char 0)) instructs Emacs to first evaluate (scroll-up 1) and then evaluate (goto-char 0). While the expressions are evaluated sequentially, progn does not introduce any explicit synchronization or waiting for redisplay operations. This lack of synchronization is the key to understanding the observed behavior.

progn is a fundamental building block in Lisp for creating compound expressions. It allows you to combine multiple operations into a single expression, which is essential for control flow constructs like if, when, and dolist. However, it's important to remember that progn itself does not alter the evaluation semantics of the individual expressions within it. It simply provides a way to group them.

In our scenario, the rapid execution of goto-char after scroll-up within the progn exacerbates the timing issue related to redisplay. The absence of any explicit delay or synchronization mechanism means that goto-char is likely to be executed before the window has visually scrolled, leading to the point moving to the wrong position.

So, how can we ensure that the point moves to the beginning of the buffer after the window has scrolled? Several approaches can be employed, each with its own trade-offs:

1. Explicit Delay

One straightforward solution is to introduce a short delay between the scroll-up and goto-char operations. This delay provides Emacs with more time to complete the redisplay triggered by scroll-up before goto-char is executed. The sleep-for function can be used to introduce such a delay.

(progn
  (scroll-up 1)
  (sleep-for 0.1) ; Wait for 0.1 seconds
  (goto-char 0))

While this approach can be effective, it's not ideal. Introducing arbitrary delays can make your code less responsive and might not be reliable across different systems or Emacs configurations. The required delay might vary depending on factors like system load and display hardware. Over-reliance on explicit delays is generally discouraged in Emacs Lisp programming.

2. redisplay Function

A more robust approach is to explicitly force a redisplay using the redisplay function. Calling redisplay ensures that Emacs updates the window immediately before proceeding with subsequent operations.

(progn
  (scroll-up 1)
  (redisplay) ; Force redisplay
  (goto-char 0))

This method is generally preferable to using explicit delays, as it directly addresses the issue of display synchronization. However, it's important to use redisplay judiciously. Excessive calls to redisplay can negatively impact performance, as they force Emacs to redraw the window more frequently than necessary. Aim to use redisplay only when it's essential to ensure that the display is up-to-date before proceeding with subsequent operations.

3. save-excursion

Another approach involves using save-excursion, which is a powerful Emacs Lisp construct for temporarily modifying the buffer state and then restoring it to its original condition. While save-excursion doesn't directly address the redisplay issue, it can be used to encapsulate the point movement operation in a way that minimizes potential side effects.

(progn
  (scroll-up 1)
  (save-excursion
    (goto-char 0)))

In this case, save-excursion ensures that the point's original position is restored after goto-char is executed. This might not be the desired behavior in all cases, but it can be useful in scenarios where you want to temporarily move the point without permanently altering its location. Understanding how save-excursion interacts with display operations can be valuable for writing complex Emacs Lisp code.

4. Custom Function or Advice

For more complex scenarios, you might consider creating a custom function or using Emacs's advice mechanism to modify the behavior of scroll-up or goto-char. For example, you could define a custom function that combines scroll-up and redisplay into a single operation.

(defun scroll-up-and-redisplay (n)
  (interactive "p")
  (scroll-up n)
  (redisplay))

(progn
  (scroll-up-and-redisplay 1)
  (goto-char 0))

Alternatively, you could use advice to modify the behavior of scroll-up to automatically trigger a redisplay. However, using advice should be done with caution, as it can have unintended side effects if not used carefully. Tailoring functions to address specific behaviors provides better control over code flow and display handling.

The seemingly simple difference between (progn (scroll-up 1) (goto-char 0)) and evaluating the expressions sequentially highlights a crucial aspect of Emacs Lisp programming: the importance of understanding the interplay between display operations and expression evaluation. The asynchronous nature of window updates and redisplay can lead to subtle timing issues that require careful consideration. By understanding the behavior of functions like scroll-up and goto-char, and by employing techniques like explicit redisplay or custom functions, you can write more robust and reliable Emacs Lisp code that interacts seamlessly with the Emacs display.

This exploration underscores the depth and richness of Emacs Lisp. Mastering these nuances is essential for becoming a proficient Emacs Lisp developer and for crafting powerful and customized Emacs environments. Remember, the key is to be mindful of the execution context and to account for potential timing issues when manipulating the display. By doing so, you can harness the full potential of Emacs Lisp to create a truly personalized and efficient editing experience.