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