Error: `error: cannot coerce a function to a string`

This article was originally published at https://gist.github.com/joepie91/b0041188c043259e6e1059d026eff301. 

Probably caused by a syntax ambiguity when invoking functions within a list. For example, the following will throw this error:

{
  # ...
  srcs = [
    fetchurl {
      url = "https://github.com/adobe/brackets-shell/archive/${shellBranch}.tar.gz";
      sha256 = shellHash;
    }
    fetchurl {
      url = "https://github.com/adobe/brackets/archive/release-${version}.tar.gz";
      sha256 = "00yc81p30yamr86pliwd465ag1lnbx8j01h7a0a63i7hsq4vvvvg";
    }
  ];
  # ...
}

This can be solved by adding parentheses around the invocations:

{
  # ...
  srcs = [
    (fetchurl {
      url = "https://github.com/adobe/brackets-shell/archive/${shellBranch}.tar.gz";
      sha256 = shellHash;
    })
    (fetchurl {
      url = "https://github.com/adobe/brackets/archive/release-${version}.tar.gz";
      sha256 = "00yc81p30yamr86pliwd465ag1lnbx8j01h7a0a63i7hsq4vvvvg";
    })
  ];
  # ...
}