# init(type:a:) Creates an activation descriptor with the activation type and parameter a that you specify. ``` convenience init?(type activationType: MLCActivationType, a: Float) ``` ## Parameters `activationType` A type of activation function. `a` Parameter a. ## Discussion Use this initializer to create one of the following activation descriptors: - CELU: `f(x) = max(0, x) + min(0, a * (exp(x / a) − 1))` Activation type: [`MLCActivationType.celu`](/documentation/MLCompute/MLCActivationType/celu) For common behavior, set `a` to `1.0`. - HardShrink: `f(x) = x`, if `x > a` or `x < −a`, else `0` Activation type: [`MLCActivationType.hardShrink`](/documentation/MLCompute/MLCActivationType/hardShrink) For common behavior, set `a` to `0.5`. - Parametric ELU: `f(x) = x >= 0 ? x : a * (exp(x) - 1)` Activation type: [`MLCActivationType.elu`](/documentation/MLCompute/MLCActivationType/elu) For common behavior, set `a` to `1.0`. - ReLU: `f(x) = x >= 0 ? x : a * x` Activation type: [`MLCActivationType.relu`](/documentation/MLCompute/MLCActivationType/relu) This is also referred to as Leaky ReLU. Some literature defines classical ReLU as `max(0, x)`. If you want this common behavior, set `a` to `0.0`. - SoftShrink: `f(x) = x - a`, if `x > a, x + a`, if `x < −a`, else `0` Activation type: [`MLCActivationType.softShrink`](/documentation/MLCompute/MLCActivationType/softShrink) For common behavior, set `a` to `0.5`. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)