Published: January 15, 2020
by Tobias Pleyer
Tags: nix

It is always good to know what you are doing

I guess nobody will object to this statement, but in this post I just want to share a mistake I made while trying to work with a software I am not yet very familiar with. The problem I want to address is that it is sometimes pretty hard for a newcomer to evaluate the information to be found on various blogs, threads, or even the official web site.

With the start of the year I also started a new job. At work we are using the Obelisk framework. Obelisk makes heavy use of Nix. Because Obelisk has a lot of dependencies it makes use of so called binary caches. That means instead of building everything locally on your machine you can download pre-built packages from a server which leads to a massive reduction of build time. Obelisk’s Github page tells you how to configure the caches. I am fairly new to Nix so I blindly did what I was told to do.

In my private time at home I started to play around with Leksah, an IDE for Haskell written in Haskell. As of lately the only supported build method for Leksah is deploying Nix. Leksah’s wiki page recommends setting up the binary caches of IOHK and refers to their Github

I quote the instructions here:

Add the following lines to /etc/nix/nix.conf. If the file does not exist,
then create it.

The word Add can be very misleading here. I interpreted add like append, so I appended the provided lines to my existing /etc/nix/nix.conf file…

Admittedly if you compare the instructions and contents of both wiki pages you probably would start to become a bit wary, but as a programmer you become so accustomed to copy-pasting stuff from the internet that it is easy to miss that point. At this point my nix.conf was broken. The problem is that most of the instructions on wiki pages assume that you had just installed Nix and nothing more. Of course it is impossible for the maintainers to know what crazy stuff the people that use their software do. BUT: Sometimes it is worth explaining a bit what the given config entries actually do. For example substituters is a whitespace separated list and multiple occurences are not ok. This can be read here for example.

Strangely though Nix seems to silently ignore this error and just default to building everything! I just came to realize something is wrong as my builds became absurdly long and no messages about cache downloads were popping up anymore.


Configurations are a pain point most of the time, because there are simply too many existing formats and standards out there. In addition you usually already must know the application in order to understand what the options are. And last but not least writing goog documentation is sadly most of the time neglected, partly because of time constraints and partly because developers usually don’t like to write documentation…