Going beyond the REPL

Improving the UX of interactive programming

REPL-driven development has long been lauded by Lisp programmers, but has failed to go mainstream. Can we find a better approach to interactive development?

The problems with REPLs

Separation of the REPL and code

REPL-driven development generally involves testing expressions in a REPL, and then transferring them to code when completed. The state of your REPL and code are completely disconnected. Some common issues here are:

Ephemeral knowledge

Exploratory development via a REPL can be incredibly empowering. Rather than guessing-and-checking via print statements, you can interactively test any expression.

But once you've finished your coding session, what happens? You've built knowledge on how all the expressions in your function operate, and after closing your REPL window…it all disappears. We just throw it away.

By giving the developer powerful introspection abilities but not the reader, REPLs create a rift in understanding. Questions that are trivially answered in the developer's state of mind are lost to the reader because they don't have the same context. Code written without a REPL is clearer to the reader by necessity, because the information the developer and reader are working are more similar.

What about debuggers?

Debuggers are great, but they don't replace REPLs. Testing an arbitrary expression requires writing it, re-running the entire program to a breakpoint, and then inspecting the value. This is too slow for REPL-driven development; we need to incrementally update the program state.

What about computational notebooks?

Computational notebooks have gained mainstream support, primarily due to their REPL-like structure. Unfortunately, they are generally single-file programs meant to run interactively, and are too narrow in scope for a general programming technique. Computational notebooks do, however, hint at the direction we need to go.

The idea

Regardless of what language you're in, there's always some way to inspect an expression. The problem is not can we test an expression, but how fast we can do so. REPLs test small expressions quickly, but introduce a friction boundary between the REPL and code. Can we test small changes directly in the code, rather than in a REPL?

f x = out
    a = op1 x
    b = op2 a
    out = some_complex_function b

-- We need to know what b is first!
-- Copying all the previous expressions is tedious
>> a = op1 "my input"
>> b = op2 a
>> some_complex_function d
"my answer"

Technologically, this is not an interesting idea; debuggers already do this. The point is developing a UX that encourages a healthier developer workflow.

How many times have you written an entire function, file, or more before ever running any code? How often were you unsure if an expression was correct, but delayed testing it until the function was complete? The harder it is to inspect and test code, the longer we will wait until testing it.

We need to put the value of an expression right in your face, as soon as you write it. Let's evaluate our code continuously, and show the values of expressions as soon as you type them.

As a developer, this makes the feedback between writing and testing code instant. There's no explicit action to take to start a test, and so no incentive to wait to test. If an expression is incorrect on the given test case, the developer will see that immediately after writing it.

As a reader, this is a powerful tool for exploring an unfamiliar codebase. Learning by example is a powerful tool, and seeing concrete instances of any value lets readers think less abstractly about code.

Hijacking unit tests

An example function with the values of all intermediate expressions shown on screen based on a unit test

We can use unit tests to run our desired function. Not only does give the reader documented examples to introspect, but it also encourages developing unit tests in parallel with function development.

Loops & recursion

If our function has recursion (or loops which can be expressed as recursion), an expression can be run multiple times in a single call. How do we let the user explore all different usages?

As long as our arguments are immutable, storing the arguments to all calls of a function is generally not significantly expensive. Then the user can selectively introspect a specific call, or see the value of an expression/return value across a series of calls.

factorial 0 = 2 -- Uh oh, this should be 1!
factorial x = x * factorial (x - 1)

-- Seeing the return values across the call stack
-- makes finding the bug easy
>> factorial 3
factorial 3 -- 12
factorial 2 -- 4
factorial 1 -- 2
factorial 0 -- 2

In pathological cases where complete storage is too expensive, we can still:

  1. Explore a single branch of recursion rather than the entire tree, which is guaranteed to be as cheap as the function call
  2. If the input range of the expression is small, wrap it in an anonymous function and observe all the times it is called

Retrofitting existing languages

We can inefficiently implement this in any functional language with a REPL via code transformation. Adding wrappers around the tested function and expressions makes it easy to track their values and arguments.

factorial x = x * factorial (x - 1)
=> factorial x = memoize 
        (Memo {args=[x], value=x * factorial (x - 1)})

But this requires re-running the entire function on change! A key feature of REPLs and computational notebooks is running an expensive expression once, and then re-using it. Can we do this automatically?

Taking advantage of purely functional code

If we know parts of our code are purely functional, then yes! Let's look at a simplified model of function design to see why.

Say a function is a series of assignments, that eventually returns an expression:

format_matrix matrix = -- some expensive computation

make_csv headers matrix = csv_output
    formatted_matrix = format_matrix matrix
    csv_lines = map (join ",") (headers : formatted_matrix)
    csv_output = join "\n" csv_lines

Now we want to change the file to be tab delimited:

format_matrix matrix = -- some expensive computation

make_csv headers matrix = csv_output
    formatted_matrix = format_matrix matrix
    csv_lines = map (join "\t") (headers : formatted_matrix)
    csv_output = join "\n" csv_lines

We know that format_matrix has no side effects, so we can skip it and just re-run the csv_lines and csv_output expressions. Or, inversely, we know we only have to re-run csv_lines and the things that depend on it.

If we develop a function top-down, we'll generally only have to re-run the last expression. This is potentially much more efficient than running a test case from scratch.

Parting thoughts

For decades, our solution to fundamental challenges with programming has been "make a new language". This is important, but I think we have neglected innovating how we interface with code. Going beyond the REPL is just one step in improving our programming methodologies.