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Add more iceberg post

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@ -720,3 +720,72 @@ bool EvalState::eqValues(Value & v1, Value & v2, const PosIdx pos, std::string_v
This "temporary hack" was commited in 14 years ago. You can do whatever
you want with this information.
## Nix Plugins
As suprising as it sounds, Nix does indeed supports plugins. You can load plugins
using the [`plugin-files`](https://nix.dev/manual/nix/2.22/command-ref/conf-file#conf-plugin-files)
configuration option.
From the configuration reference:
> A list of plugin files to be loaded by Nix. Each of these files will be dlopened by
> Nix. If they contain the symbol nix_plugin_entry(), this symbol will be called.
> Alternatively, they can affect execution through static initialization. In particular,
> these plugins may construct static instances of RegisterPrimOp to add new primops
> or constants to the expression language, RegisterStoreImplementation to add new
> store implementations, RegisterCommand to add new subcommands to the nix command,
> and RegisterSetting to add new nix config settings. See the constructors for those
> types for more details.
>
> Warning! These APIs are inherently unstable and may change from release to release.
>
> Since these files are loaded into the same address space as Nix itself, they must
> be DSOs compatible with the instance of Nix running at the time (i.e. compiled
> against the same headers, not linked to any incompatible libraries). They should
> not be linked to any Nix libs directly, as those will be available already at load time.
>
> If an entry in the list is a directory, all files in the directory are loaded
> as plugins (non-recursively).
Some example plugins are [`nix-doc`](https://github.com/lf-/nix-doc)
and [`nix-extra-builtins`](https://github.com/shlevy/nix-plugins).
## `/bin/sh` and sandbox impurity
By setting the [`sandbox-paths`](https://nix.dev/manual/nix/2.22/command-ref/conf-file#conf-sandbox-paths)
option to `/bin/sh=/bin/sh`, Nix will bind the `/bin/sh` path in the
build sandbox (left) to the `/bin/sh` path in the host (right).
This is of course impure, but is useful for bootstrapping from
absolute scratch without copying impure binaries to the Nix store.
## `rec { a = 5; b = a + 1; __overrides.a = 6; }`
There is a special field named `__overrides` in recursive attrset expressions,
which simply overrides the parent attribute set with the keys inside it. This
is different from the update operator (`//`) because that will not override the
self-referneces in the recursive attribute set.
`rec { a = 5; b = a + 1; __overrides.a = 6; }.b` will evaluate to 7,
while `(rec { a = 5; b = a + 1; } // { a = 6; }).b` will evaluate to 6.
## `let __div = c: map (__mul c); in 2 / [ 1 2 3 ]`
As mentioned in my [HTMNIX blog post](/blog/htmnix), Nix operators get
desugared into normal function calls before execution. All operators
have their "hidden" equivalents that they get desugared into (`__div` is for `/`, etc.),
so you can override them using `let in`.
`let __div = c: map (__mul c); in 2 / [ 1 2 3 ]` is equivalent to
`map (x: 2 * x) [ 1 2 3 ]` which evaluates to `[ 2 4 6 ]`.
You can also check what a Nix snippet desugars into
using `nix-instantiate --parse --expr 'expression here'`
## `let __lessThan = a: b: b - a; in 1 > 2`
As mentioned above, this expression will desugar into
`let __lessThan = a: b: b - a; in __lessThan 1 2` which
will evaluate to 1.
## `__impure`