med ∩ ml

Fixing a bug in the Haskell installer

The story of a bug fix. I found a bug in the default Haskell installer ghcup, fixed it in my computer and had the fix merged to the public installer.

I started doing the MIT course Programming with Categories, so I had to install Haskell on my computer. But following the recommended method kept throwing me an error: mktemp: too few X's in template ‘ghcup’. After some time searching I could only find 1 discussion where someone had had a similar problem.

After learning a bit about mktemp and digging into the installer script, I discovered that different versions of mktemp had different behaviors. In my case I had two installed, the one coming by default with macOS, placed in /usr/bin and a different one installed by brew when installing coreutils.

I modified the installer script like so:

mktemp -d -t ghcup => mktemp -d -t ghcup.XXXXXXX

Everything suddenly worked. I opened an issue and a pull request in the ghcup repository, and a few hours later it was merged!

The only problem we could think of was breaking the installer for other macOS / OS X versions, and that’s something I also asked myself at first.

My guess is that “ghcup.XXXXXXX” would be evaluated as a string anyway, the same way “ghcup” alone is evaluated. I’m currently not planning to upgrade to the latest macOS version, so I couldn’t test it with macOS Catalina or with older versions. I made some tests on my computer though:

Using native mktemp, the one that comes with macOS (/usr/bin/mktemp):

  • /usr/bin/mktemp -d -t ghcup.XXXX creates a folder named: ghcup.XXXX.r2OBbQRF

  • /usr/bin/mktemp -d -t ghcupXXXX (without the dot) creates a folder named: ghcupXXXX.wEPBMrGh

  • /usr/bin/mktemp -d -t ghcup (the one that runs in macOS without coreutils installed) creates a folder named: ghcup.8fAcY5LC

Using coreutils (GNU) mktemp v. 8.31, the default one in my computer, installed with homebrew (/usr/local/opt/coreutils/libexec/gnubin/mktemp):

  • mktemp -d -t ghcup.XXXX creates a folder named: ghcup.7Gwj

  • mktemp -d -t ghcup.XXXX (without the dot) creates a folder named: ghcup3jJS

  • mktemp -d -t ghcup(the one throwing the error) returns: mktemp: too few X’s in template ‘ghcup’

After the merge I tried installing Haskell with the default installation method/command and boom 💥, everything worked smoothly.

I know this is a very subtle bug that did not require extensive research for months to fix, but maybe someone else had, could, or will run into the same problem; and thinking that this could be the fix for it makes me feel fantastic 😄.