# `Ichor.Backtrack.Term`
[🔗](https://github.com/joetjen/ichor_runtime/blob/v0.2.0/lib/ichor/backtrack/term.ex#L1)

The behaviour a caller's own term representation implements so
`Ichor.Backtrack.Bindings` can unify it -- passed explicitly to every
`Bindings` function (`resolve/3`, `unify/4`), the same "module passed
as a plain argument, never looked up dynamically" convention
`Ichor.Actions.evaluate/5` already uses for an actions module.

Nothing here prescribes what a term actually *is* -- a caller is free
to use plain Elixir terms directly for anything atomic (`:tom`, `42`,
`"a string"`, compared with `==/2`), reserving a distinguishing shape
only for the two cases that need special handling:

  - `variable?/1` + `var_id/1` -- a logic variable, keyed by whatever
    `var_id/1` returns (this is exactly the key `Bindings` stores a
    binding under, so it must be stable and unique per distinct
    variable -- an Elixir `reference/0` minted fresh per variable is
    a natural choice).
  - `compound?/1` + `deconstruct/1` -- a compound term (`foo(X, bar)`),
    unified by recursively unifying its functor and arguments
    (`deconstruct/1` returning `{functor, args}`); two compound terms
    only unify when their functor and arity both match.

Anything that's neither a variable nor a compound term is treated as
atomic and unified with plain `==/2`.

`reconstruct/2` is optional -- the inverse of `deconstruct/1`, needed
only by callers that *rebuild* terms rather than just inspect them
(`Ichor.Toolkit.TypeScheme`'s `generalize/4`/`instantiate/3`, which
substitute type variables inside a type). Unification itself
(`Ichor.Backtrack.Bindings`) never needs it -- it only ever compares
and binds, never constructs.

# `compound?`

```elixir
@callback compound?(term()) :: boolean()
```

# `deconstruct`

```elixir
@callback deconstruct(term()) :: {functor :: term(), args :: [term()]}
```

# `reconstruct`
*optional* 

```elixir
@callback reconstruct(functor :: term(), args :: [term()]) :: term()
```

# `var_id`

```elixir
@callback var_id(term()) :: term()
```

# `variable?`

```elixir
@callback variable?(term()) :: boolean()
```

---

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