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docs/HACKING.md

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+# HACKING
+
+## Design decisions
+
+### Using [Excepts](https://hackage.haskell.org/package/haskus-utils-variant-3.0/docs/Haskus-Utils-Variant-Excepts.html) as a beefed up ExceptT
+
+This is an open variant, similar to [plucky](https://hackage.haskell.org/package/plucky) or [oops](https://github.com/i-am-tom/oops) and allows us to combine different error types. Maybe it is too much and it's a little bit [unergonomic](https://github.com/haskus/packages/issues/32) at times. If it really hurts maintenance, it will be removed. It was more of an experiment.
+
+### No use of haskell-TLS
+
+I consider haskell-TLS an interesting experiment, but not a battle-tested and peer-reviewed crypto implementation. There is little to no research about what the intricacies of using haskell for low-level crypto are and how vulnerable such binaries are. Instead, we use either curl the binary (for FreeBSD and mac) or http-io-streams, which works with OpenSSL bindings.
+
+### Optics instead of lens
+
+They're a little safer (less Monoid weirdness with view) and have better error messages. Consider the following wit lens
+
+```
+> view (_Just . to (++ "abc")) Nothing
+""
+```
+
+vs optics
+
+```
+> view (_Just % to (++ "abc")) Nothing
+
+<interactive>:2:1: error:
+    • An_AffineFold cannot be used as A_Getter
+    • In the expression: view (_Just % to (++ "abc")) Nothing
+      In an equation for ‘it’: it = view (_Just % to (++ "abc")) Nothing
+```
+
+### Strict and StrictData on by default
+
+Kazu Yamamoto [explained it in his PR](https://github.com/yesodweb/wai/pull/752#issuecomment-501531386) very well. I like to agree with him. The instances where we need non-strict behavior, we annotate it.
+
+## Code style and formatting
+
+1. Brittany
+2. mtl-style preferred
+3. no overly pointfree style
+
+## Code structure
+
+Main functionality is in `GHCup` module. Utility functions are
+organised tree-ish in `GHCup.Utils` and `GHCup.Utils.*`.
+
+Anything dealing with ghcup specific directories is in
+`GHCup.Utils.Dirs`.
+
+Download information on where to fetch bindists from is in the appropriate
+yaml files: `ghcup-<yaml-ver>.yaml`.
+
+## Common Tasks
+
+### Adding a new GHC version
+
+1. open the latest `ghcup-<yaml-ver>.yaml`
+2. find the latest ghc version (in yaml tree e.g. `ghcupDownloads -> GHC -> 8.10.3`)
+3. copy-paste it
+4. adjust the version, tags, changelog, source url
+5. adjust the various bindist urls (make sure to also change the yaml anchors)
+6. run `cabal run exe:ghcup-gen -- check-tarballs -f ghcup-<yaml-ver>.yaml -u 'ghc-8\.10\.4'`
+
+## Major refactors
+
+1. First major refactor included adding cross support. This added
+   `GHCTargetVersion`, which includes the target in addition to the version.
+   Most of the `Version` parameters to functions had to be replaced with
+   that and ensured the logic is consistent for cross and non-cross
+   installs.
+2. This refactor added windows support wrt [#130](https://gitlab.haskell.org/haskell/ghcup-hs/-/issues/130).
+   The major changes here were switching `hpath` library out for `filepath`/`directory` (sadly) and
+   introducing a non-unix way of handling processes via the `process` library. It also introduced considerable
+   amounts of CPP wrt file handling, installation etc.