elaforge/gist:15cb35067ce81b44c81ca429a16f25b9

## gistfile1.txt
. It would be nice to include guidance on how to pick a nixpkgs commit.
I went to github and picked the latest from nixos-19.09.  Then how do we know
which version of ghc it has?  I did `nix repl` then `n = import nixpkgs.nix {}`
then use n.<tab> to see what's inside, but that's clumsy.  Surely there's some
better way?

. I used `fetchTarball` to get nixpkgs, not `fetchGit`.  fetchGit locked up my
laptop... probably it's just slower and nixpkgs is repo.

. I have to pass --option max-jobs 4 (less than default of 32) or it kills the
laptop by trying to run too much at once.

. `-L` gets `nix build` to print the build logs, so you can see what it's
doing.

. `nixpkgs.nix-prefetch-git` doesn't have `prefetch-url` as the post suggests.
The proper name is nix-prefetch-url, and it seems to be included with nix itself.

. The overlay didn't seem to have an effect, but my old packageOverrides thing
does work.

. You have to decide on the package versions to use in the first place, which
means you need `cabal freeze`.  So I think the workflow has to be, write the
cabal file, then `cabal freeze`, then parse the `cabal.config`.  Since it's not
practical to manually get hashes for more than a few packages, I wrote a script
to do this.  I haven't looked into the v2 freeze file yet, it's significantly
different because it also encodes the build flags.  These probably need to be
propagated to callCabal2Nix.

. Here's the big thing though: it turns out that `callHackageDirect` is broken.
The problem is that it doesn't take metadata revisions into account, so you run
into an unbuildable package pretty soon.  Since the versions are generated by
cabal, it should be possible to doJailbreak, but I discovered that
`jailbreak-cabal` is also unreliable, because it misses versions in flags.
From a look at cabal docs, "--allow-older --allow-newer" is the real way to do
this, but apparently this is brand new, because while 3.2.0.0 has it, 3.0.1.0
apparently doesn't.  Since Cabal is a bootlib, it's locked to the ghc version.

So, we need a better callHackageDirect.  Actually it wasn't hard to write one,
since I found out you can download patched cabal files from the
`all-cabal-files` github repo.  I modified the sha-fetching script to fetch two
hashes and wrote a derivation to combine them, and then call `callCabal2Nix` on
that.

I still need to patch some packages which are broken in nixpkgs: zlib and
digest at least.  But after that it seems like it actually works, though
I haven't done the plumbing yet to actually compile with these packages.
	. It would be nice to include guidance on how to pick a nixpkgs commit.
	I went to github and picked the latest from nixos-19.09. Then how do we know
	which version of ghc it has? I did `nix repl` then `n = import nixpkgs.nix {}`
	then use n.<tab> to see what's inside, but that's clumsy. Surely there's some
	better way?

	. I used `fetchTarball` to get nixpkgs, not `fetchGit`. fetchGit locked up my
	laptop... probably it's just slower and nixpkgs is repo.

	. I have to pass --option max-jobs 4 (less than default of 32) or it kills the
	laptop by trying to run too much at once.

	. `-L` gets `nix build` to print the build logs, so you can see what it's
	doing.

	. `nixpkgs.nix-prefetch-git` doesn't have `prefetch-url` as the post suggests.
	The proper name is nix-prefetch-url, and it seems to be included with nix itself.

	. The overlay didn't seem to have an effect, but my old packageOverrides thing
	does work.

	. You have to decide on the package versions to use in the first place, which
	means you need `cabal freeze`. So I think the workflow has to be, write the
	cabal file, then `cabal freeze`, then parse the `cabal.config`. Since it's not
	practical to manually get hashes for more than a few packages, I wrote a script
	to do this. I haven't looked into the v2 freeze file yet, it's significantly
	different because it also encodes the build flags. These probably need to be
	propagated to callCabal2Nix.

	. Here's the big thing though: it turns out that `callHackageDirect` is broken.
	The problem is that it doesn't take metadata revisions into account, so you run
	into an unbuildable package pretty soon. Since the versions are generated by
	cabal, it should be possible to doJailbreak, but I discovered that
	`jailbreak-cabal` is also unreliable, because it misses versions in flags.
	From a look at cabal docs, "--allow-older --allow-newer" is the real way to do
	this, but apparently this is brand new, because while 3.2.0.0 has it, 3.0.1.0
	apparently doesn't. Since Cabal is a bootlib, it's locked to the ghc version.

	So, we need a better callHackageDirect. Actually it wasn't hard to write one,
	since I found out you can download patched cabal files from the
	`all-cabal-files` github repo. I modified the sha-fetching script to fetch two
	hashes and wrote a derivation to combine them, and then call `callCabal2Nix` on
	that.

	I still need to patch some packages which are broken in nixpkgs: zlib and
	digest at least. But after that it seems like it actually works, though
	I haven't done the plumbing yet to actually compile with these packages.