# `Tptp.Resolver.Fs`
[🔗](https://github.com/jcschuster/tptp/blob/v0.1.3/lib/tptp/resolver/fs.ex#L1)

Resolves an `include` against the local filesystem.

## Search order

1. the directory of the including file, when there is one;
2. `:root`, or `$TPTP_ROOT` if the option is absent;
3. `$TPTP`, the older variable the TPTP tools have always used;
4. `:cwd`, or the current working directory.

`$TPTP_ROOT` is the documented environment variable and `:root` is the local
override, so that a caller directing resolution at a vendored copy, a fixture
directory or a second library need not set an environment variable for the whole
VM.

    Tptp.Unit.from_file("PUZ001+1.p", resolver: Tptp.Resolver.Fs)
    Tptp.Unit.from_file("PUZ001+1.p", resolver: {Tptp.Resolver.Fs, root: "/opt/TPTP"})

`:root` takes a list too, tried in order, for a caller layering a local override
over a shared library:

    {Tptp.Resolver.Fs, root: ["priv/my_axioms", "/opt/TPTP"]}

`:root` does not suppress `$TPTP` or the working directory; it is inserted ahead
of them. To search only the named directories, pass `cwd: false` and leave the
environment variables unset, or use `Tptp.Resolver.Map`.

`$TPTP` is honoured after `:root` because a machine with the TPTP distribution
installed usually already has it set, and failing to find `Axioms/SET007+0.ax` on
such a machine would be a silly way to lose.

## Confinement

An include name originates in a file that may not be trusted, so it must be a
relative path that does not ascend; see `Tptp.Resolver.safe?/1`. The name is
checked rather than the joined result, which makes the condition independent of
the candidate directory: no absolute paths and no `..`, in any position.

## Canonicalisation

The returned path is expanded, so two routes to one file — `Axioms/a.ax` from the
root and `a.ax` from the `Axioms` directory — memoise to the same entry and a
diamond in the include graph is read once.

# `roots`

```elixir
@spec roots(
  Path.t() | nil,
  keyword()
) :: [Path.t()]
```

The directories this resolver would look in, in order.

Exposed because "it could not find the file" is a much less useful thing to be
told than "it looked here, here and here".

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*Consult [api-reference.md](api-reference.md) for complete listing*