# `EXLA.CustomCall.Spec`
[🔗](https://github.com/elixir-nx/nx/blob/v0.12.0/exla/lib/exla/custom_call/spec.ex#L1)

Result of `EXLA.CustomCall.call/4` when lowering a tagged `Nx.block/4` to
`stablehlo.custom_call`.

  * **`call_target_name`** — XLA FFI handler name (`call_target_name` on the op).

  * **`attributes`** — Optional `{name, attr}` pairs, default `[]`, merged into
    the `backend_config` dictionary on `stablehlo.custom_call` (StableHLO’s name
    for that attribute). Each `name` must be a **BEAM binary** MLIR identifier; each
    `attr` must be a **BEAM binary** with valid MLIR attribute syntax for the RHS after
    `name = ` (for example `{"k", "42 : i64"}`). An empty list omits the dictionary
    from the op.

  * **`operand_element_types`** — How operand SSA values are presented to the handler:

    * **`:default`** — use each lowered operand’s element type as produced from the
      block inputs. No extra converts.

    * **`[Nx.Type.t(), ...]`** — one type per block input, same order and length as
      `Nx.block/4` inputs. Before building the custom call, each operand is
      converted (StableHLO `convert`) when its element type differs from the
      requested type; shapes are unchanged. Use this when the native kernel’s
      FFI signature expects dtypes that may differ from the traced expression
      types (for example after promotion rules).

# `t`

```elixir
@type t() :: %EXLA.CustomCall.Spec{
  attributes: [{String.t(), String.t()}],
  call_target_name: String.t(),
  operand_element_types: :default | [Nx.Type.t()]
}
```

---

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