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readTree
========
This is a Nix program that builds up an attribute set tree for a large
repository based on the filesystem layout.
It is in fact the tool that lays out the attribute set of this repository.
As an example, consider a root (`.`) of a repository and a layout such as:
```
.
├── third_party
│ ├── default.nix
│ └── rustpkgs
│ ├── aho-corasick.nix
│ └── serde.nix
└── tools
├── cheddar
│ └── default.nix
└── roquefort.nix
```
When `readTree` is called on that tree, it will construct an attribute set with
this shape:
```nix
{
tools = {
cheddar = ...;
roquefort = ...;
};
third_party = {
# the `default.nix` of this folder might have had arbitrary other
# attributes here, such as this:
favouriteColour = "orange";
rustpkgs = {
aho-corasick = ...;
serde = ...;
};
};
}
```
Every imported Nix file that yields an attribute set will have a `__readTree =
true;` attribute merged into it.
## Traversal logic
`readTree` will follow any subdirectories of a tree and import all Nix files,
with some exceptions:
* A folder can declare that its children are off-limit by containing a
`.skip-subtree` file. Since the content of the file is not checked, it can be
useful to leave a note for a human in the file.
* If a folder contains a `default.nix` file, no *sibling* Nix files will be
imported - however children are traversed as normal.
* If a folder contains a `default.nix` it is loaded and, if it evaluates to a
set, *merged* with the children. If it evaluates to anything else the children
are *not traversed*.
Traversal is lazy, `readTree` will only build up the tree as requested. This
currently has the downside that directories with no importable files end up in
the tree as empty nodes (`{}`).
## Import structure
`readTree` is called with two parameters: The arguments to pass to all imports,
and the initial path at which to start the traversal.
The package headers in this repository follow the form `{ pkgs, ... }:` where
`pkgs` is a fixed-point of the entire package tree (see the `default.nix` at the
root of the depot).
In theory `readTree` can pass arguments of different shapes, but I have found
this to be a good solution for the most part.
Note that `readTree` does not currently make functions overridable, though it is
feasible that it could do that in the future.
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