Ylan Segal

Fast Feedback Loops

One of the reasons that I love TDD, is that it promotes fast feedback. You write a line, execute the tests, and see what the results are. I write outside-in-TDD most of the time. Occasionally, I don’t have a clear idea of what tests to write, or I am doing exploratory coding.

For example, lately I’ve found myself writing a fair amount of raw SQL queries (without an ORM). SQL is finicky, and produced notoriously hard-to-decipher errors. As a consequence, I like to build up SQL in small increments, and execute the work-in-progress statement often, to see it and its output alongside each other. My workflow looks something like this:

What is going on? I selected some SQL, executed in psql, and appended the commented-out output into the same selection. After inspection, I can change the statement and repeat.

Why?

The benefit I get from this workflow is that I can iterate in small steps, get feedback on what the current code does, and continue accordingly. This workflow is heavily inspired by Ruby’s xmpfilter or the newer seeing_is_believing. Both tools take Ruby code as input, execute it, and then record all (or some) of the evaluated code as comments to the code.

How?

This workflow is made possible by leveraging the pipe Atom package. I previously described it. It allows sending the current selection in Atom to any Unix command (or series of piped commands) and replaces the selection with the output.

Building on top of that, I wanted a unix command (that I called io, for lack of imagination) that would output both the original input and the commented-out output:

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#!/usr/bin/env bash
# Prints stdin and executes in the given program, commenting the output.

set -euo pipefail

# Determine which comment pattern to use
case $1 in
  psql)
    comment='-- ' ;;
  *)
    comment='# ' ;;
esac

grep -v "^$comment" /dev/stdin | tee >("$@" | sed "s/^/$comment/")

The case statement selects the correct comment prefix. It is customary in many Unix tools to treat a line starting with # as a comment. psql is different, in that it uses -- prefix. I haven’t needed support for anything else, but it’s easily extendible.

The meat of the execution breaks down like this:

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# Reads /dev/stdin and removes and lines starting with a comment
grep -v "^$comment" /dev/stdin

# The comment-less input, is now send to tee.
# tee will redirect the input to a file and to stdout.
| tee

# Instead of a file, we give tee a sub-shell as a file descriptor, using
# process substition.
>( )

# That subshell will execute the rest of the arguments passed to io
# as a command
"$@"

# the output is piped to sed, to add the comment prefix to every line
| sed "s/^/$comment/"

The result is that the final output is what we have been looking for: The original input without comments, plus the executed input with comments added.

From my point of view, this is a great example of the Unix philosophy: Composing utilities to create new functionality. I took advantage of the flexibility in input/output redirection and process substitution to improve my development workflow.