I added the version as the date today because the package doesn't have versions. Unless you count the paper 1999 as the "version". I'm not even sure if this paper: LeCun, Y., Bottou, L., Bengio, Y., and Haffner, P. (1998). Gradient-based learning applied to document recognition. Proceedings of the IEEE, 86, 2278–2324.

Related: #46922

I'm also wondering if this is the right way of doing multi-source derivations. I've just copied the style from Ocaml derivations. I also wonder about mv vs cp operations in the install phase. The files are not meant to be unpacked, as MNIST readers expect the data as compressed.

I've discovered another way to create this. It's actually more minimal. Because the current derivation leaves the "srcs" still in the /nix/store, and ends up copying the data.

The below just creates symlinks to the individual data.

  let
    srcs = {
      train-images = fetchurl {
        url = "http://yann.lecun.com/exdb/mnist/train-images-idx3-ubyte.gz";
        sha256 = "029na81z5a1c9l1a8472dgshami6f2iixs3m2ji6ym6cffzwl3s4";
      };
      train-labels= fetchurl {
        url = "http://yann.lecun.com/exdb/mnist/train-labels-idx1-ubyte.gz";
        sha256 = "0p152200wwx0w65sqb65grb3v8ncjp230aykmvbbx2sm19556lim";
      };
      test-images= fetchurl {
        url = "http://yann.lecun.com/exdb/mnist/t10k-images-idx3-ubyte.gz";
        sha256 = "1rn4vfigaxn2ms24bf4jwzzflgp3hvz0gksvb8j7j70w19xjqhld";
      };
      test-labels= fetchurl {
        url = "http://yann.lecun.com/exdb/mnist/t10k-labels-idx1-ubyte.gz";
        sha256 = "1imf0i194ndjxzxdx87zlgn728xx3p1qhq1ssbmnvv005vwn1bpp";
      };
    };
  in
    linkFarm
    "mnist-2018-11-16"
    [
      { name = srcs.train-images.name; path = srcs.train-images; }
      { name = srcs.train-labels.name; path = srcs.train-labels; }
      { name = srcs.test-images.name; path = srcs.test-images; }
      { name = srcs.test-labels.name; path = srcs.test-labels; }
    ]

However I suspect this does not fit the conventions because there is no maintainer information nor is there any version information. Is there something equivalently simple? Or should I just swap the cp for ln?

@c0bw3b What do you think of using ln vs cp?

I think it's preferable to ln over cp since fetchurl is going to install the sources in the nix store.

The only other approach I can think of would be to use fetchzip instead of fetchurl and install -D -t $out <theFlatFilesWeWant> in your install phase?
You may need to add unpackPhase to you list of phases for this to work. I have not tested locally.
migmix font seems to be close enough to what I have in mind.

The files should not be decompressed.

Oh.. Then I guess your current derivation is the better approach.

@c0bw3b ready to merge?

Would it make sense to mark it with preferLocalBuild = true?

@veprbl why would we do that when none of the data packages do that?