# `StatifierBlocks.Assignability.Relation`
[🔗](https://github.com/riddler/statifier_blocks/blob/v0.44.1/lib/statifier_blocks/assignability/relation.ex#L1)

A host's widening relation. Consulted only after identity has failed, so
an implementation can only widen, never narrow (ADR-0003 decision 6).

Pure, under ADR-0002 decision 4: no process dictionary, no application
configuration, no IO, no clock, no randomness. `assignable?/2` is called
during validation on every edit, so anything impure here breaks the
determinism the compiler promises.

# `assignable?`

```elixir
@callback assignable?(
  produced :: StatifierBlocks.Assignability.type_expr(),
  consumed :: StatifierBlocks.Assignability.type_expr()
) :: boolean()
```

Does `produced` widen to satisfy `consumed`? Called only when
`produced == consumed` has already failed (ADR-0003 decision 6), so a
module answering `true` here can only grow the accepted set, never
shrink it.

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