MLX Swift Framework

MLX Swift is Apple's high-performance machine learning framework designed specifically for Apple Silicon. It provides NumPy-like array operations with lazy evaluation, automatic differentiation, and unified CPU/GPU memory.

When to Use This Skill

• Array operations on Apple Silicon (MLXArray)
• Building neural networks (MLXNN)
• Training models with automatic differentiation
• Custom Metal kernels via MLXFast
• Performance optimization with JIT compilation

Architecture Overview

MLXOptimizers (Adam, AdamW, SGD, etc.)
       ↓
MLXNN (Layers, Modules, Losses)
       ↓
MLX (Arrays, Ops, Transforms, FFT, Linalg, Random)
       ↓
Cmlx (C/C++ bindings, Metal GPU)

Key File Reference

Purpose	File Path
Core array	Source/MLX/MLXArray.swift
Operations	Source/MLX/Ops.swift
Transforms	Source/MLX/Transforms.swift
Factory methods	Source/MLX/Factory.swift
Neural layers	Source/MLXNN/*.swift
Optimizers	Source/MLXOptimizers/Optimizers.swift
Fast ops	Source/MLX/MLXFast.swift
Custom kernels	Source/MLX/MLXFastKernel.swift
Wired memory coordinator	Source/MLX/WiredMemory.swift
GPU working-set helper	Source/MLX/GPU+Metal.swift

Quick Start

Basic Array Creation

import MLX

// Create arrays
let a = MLXArray([1, 2, 3, 4])
let b = MLXArray(0 ..< 12, [3, 4])  // Shape [3, 4]
let c = MLXArray.zeros([2, 3])
let d = MLXArray.ones([4, 4], dtype: .float32)

// Random arrays (use MLXRandom namespace or free functions)
let uniform = MLXRandom.uniform(0.0 ..< 1.0, [3, 3])
let normal = MLXRandom.normal([100])

Array Properties

let array = MLXArray(0 ..< 12, [3, 4])
array.shape    // [3, 4]
array.ndim     // 2
array.size     // 12
array.dtype    // .int64
array.count    // 3 (first dimension)

Basic Operations

let a = MLXArray([1.0, 2.0, 3.0])
let b = MLXArray([4.0, 5.0, 6.0])

// Arithmetic (lazy - not computed until eval)
let sum = a + b
let product = a * b
let matmul = a.matmul(b.T)

// Force evaluation
eval(sum, product)
// or
sum.eval()

Building a Neural Network

import MLX
import MLXNN

class MLP: Module, UnaryLayer {
    @ModuleInfo var fc1: Linear
    @ModuleInfo var fc2: Linear

    init(inputDim: Int, hiddenDim: Int, outputDim: Int) {
        self.fc1 = Linear(inputDim, hiddenDim)
        self.fc2 = Linear(hiddenDim, outputDim)
        super.init()
    }

    func callAsFunction(_ x: MLXArray) -> MLXArray {
        var x = fc1(x)
        x = relu(x)
        return fc2(x)
    }
}

let model = MLP(inputDim: 784, hiddenDim: 256, outputDim: 10)
eval(model)  // Initialize parameters

Training Loop

import MLXOptimizers

let model = MLP(inputDim: 784, hiddenDim: 256, outputDim: 10)
let optimizer = Adam(learningRate: 0.001)

func loss(model: MLP, x: MLXArray, y: MLXArray) -> MLXArray {
    let logits = model(x)
    return crossEntropy(logits: logits, targets: y, reduction: .mean)
}

// Compute loss and gradients - valueAndGrad returns a function
let lossAndGrad = valueAndGrad(model: model, loss)
let (lossValue, grads) = lossAndGrad(model, x, y)

// Update model
optimizer.update(model: model, gradients: grads)
eval(model, optimizer)

Primary Workflow: Array Operations

See arrays.md for detailed array creation and indexing.

Creation Functions

// Zeros and ones
MLXArray.zeros([3, 4])
MLXArray.ones([2, 2], dtype: .float16)

// Ranges
arange(0, 10, 2)           // [0, 2, 4, 6, 8]
linspace(0.0, 1.0, 5)      // [0.0, 0.25, 0.5, 0.75, 1.0]

// Identity and diagonal
MLXArray.identity(3)
diagonal(array, offset: 0)

// Full
MLXArray.full([2, 3], values: 7.0)

Indexing

let a = MLXArray(0 ..< 12, [3, 4])

// Single element
a[0, 1]

// Slicing
a[0...]           // All rows
a[..<2]           // First 2 rows
a[1..., 2...]     // From row 1, column 2 onwards

// Advanced indexing
a[.ellipsis, 0]       // First column of all dimensions
a[.newAxis, .ellipsis]  // Add dimension at front

Shape Manipulation

let a = MLXArray(0 ..< 12, [3, 4])

a.reshaped([4, 3])
a.reshaped(-1, 6)     // Infer first dimension
a.T                    // Transpose
a.transposed(1, 0)     // Explicit transpose
a.squeezed()           // Remove size-1 dimensions
a.expandedDimensions(axis: 0)

Secondary Workflow: Neural Networks

See neural-networks.md for complete layer reference.

Built-in Layers

// Linear layers
Linear(inputDim, outputDim, bias: true)
Bilinear(in1, in2, out)

// Convolutions
Conv1d(inputChannels, outputChannels, kernelSize: 3)
Conv2d(inputChannels, outputChannels, kernelSize: 3, stride: 1, padding: 1)

// Normalization
LayerNorm(dimensions)
RMSNorm(dimensions)
BatchNorm(featureCount)
GroupNorm(groupCount, dimensions)

// Attention
MultiHeadAttention(dimensions: 512, numHeads: 8)

// Recurrent
RNN(inputSize, hiddenSize)
LSTM(inputSize, hiddenSize)
GRU(inputSize, hiddenSize)

// Regularization
Dropout(p: 0.1)

Module Property Wrappers

class MyLayer: Module {
    @ModuleInfo var layer: Linear           // Tracked module
    @ModuleInfo(key: "w") var weights: Linear  // Custom key

    let constant: MLXArray  // NOT tracked (no wrapper)
}

Loss Functions

crossEntropy(logits: logits, targets: targets, reduction: .mean)
binaryCrossEntropy(logits: logits, targets: targets)
l1Loss(predictions: predictions, targets: targets, reduction: .mean)
mseLoss(predictions: predictions, targets: targets, reduction: .mean)
smoothL1Loss(predictions: predictions, targets: targets, beta: 1.0)
klDivLoss(inputs: inputs, targets: targets, reduction: .mean)

Tertiary Workflow: Training

See transforms.md for automatic differentiation details.

Gradient Computation

// Simple gradient
let gradFn = grad { x in
    sum(x * x)
}
let g = gradFn(MLXArray([1.0, 2.0, 3.0]))

// Value and gradient together
let (value, gradient) = valueAndGrad { x in
    sum(x * x)
}(MLXArray([1.0, 2.0, 3.0]))

// Model gradients - valueAndGrad returns a function, call it to get results
let lossAndGradFn = valueAndGrad(model: model) { model in
    model(input)
}
let (loss, grads) = lossAndGradFn(model)

Optimizers

See optimizers.md for all optimizers.

// Common optimizers
let sgd = SGD(learningRate: 0.01, momentum: 0.9)
let adam = Adam(learningRate: 0.001, betas: (0.9, 0.999))
let adamw = AdamW(learningRate: 0.001, weightDecay: 0.01)

// Training step
optimizer.update(model: model, gradients: grads)
eval(model, optimizer)

Compilation for Performance

// Compile a pure array function for faster execution
let compiledOp = compile { (a: MLXArray, b: MLXArray) -> MLXArray in
    let x = a + b
    return sum(x * x)
}

// Use compiled version
let output = compiledOp(arrayA, arrayB)

// Note: compile() works best with pure MLXArray functions.
// For models, call model methods directly (they can use internal compilation).

Quaternary Workflow: Wired Memory Coordination

See wired-memory.md for full policy, hysteresis, and admission guidance.

import MLX

let policy = WiredSumPolicy()

// Reservation: participates in admission but does not keep the wired limit high while idle.
let weightsTicket = policy.ticket(size: weightsBytes, kind: .reservation)
_ = await weightsTicket.start()

// Active work: raises limit while inference runs.
let inferenceTicket = policy.ticket(size: kvCacheBytes, kind: .active)
try await inferenceTicket.withWiredLimit {
    // run model inference
}

_ = await weightsTicket.end()

Best Practices

DO

• Use lazy evaluation: MLX arrays are computed lazily. Call eval() strategically to control memory and compute.
• Batch eval calls: eval(a, b, c) is more efficient than separate calls.
• Use @ModuleInfo for all module properties to enable quantization and updates.
• Use actors for concurrent code: Encapsulate MLX state within actors for thread safety.
• Use namespaced functions: MLXRandom.uniform(), FFT.fft(), Linalg.inv().
• Use ticket-based wired memory coordination: Prefer WiredMemoryTicket.withWiredLimit and WiredMemoryManager.shared.

DON'T

• Don't share MLXArrays across tasks: MLXArray is NOT Sendable by design.
• Don't use deprecated module imports: Use import MLX not import MLXRandom.
• Don't forget to eval(): Unevaluated arrays can accumulate large compute graphs.
• Don't mutate arrays directly: Use operations that return new arrays.
• Don't call deprecated wired-limit APIs: Avoid GPU.withWiredLimit(...) and Memory.withWiredLimit(...).

Deprecated Patterns

If you see...	Use instead...
import MLXRandom	import MLX then MLXRandom.uniform() or free function uniform()
import MLXFFT	import MLX then FFT.fft()
import MLXLinalg	import MLX then Linalg.inv()
GPU.activeMemory	Memory.activeMemory
GPU.withWiredLimit(...)	WiredMemoryTicket(...).withWiredLimit { ... } via WiredMemoryManager
Memory.withWiredLimit(...)	WiredMemoryTicket(...).withWiredLimit { ... }
repeat(_:count:)	repeated(_:count:)
addmm()	addMM()
LogSoftMax	LogSoftmax
SoftMax	Softmax

See deprecated.md for the complete migration guide.

Swift Concurrency Notes

MLX has specific concurrency behavior:

• MLXArray is NOT Sendable: This is intentional. Arrays contain references to compute graphs.
• evalLock protects eval/stream creation: The global lock serializes evaluation and stream operations.
• Lazy operations are NOT thread-safe: Don't share arrays across tasks without proper synchronization.
• Use actors to encapsulate MLX state: Create and use MLXArrays within the same actor.
• Use wired-memory tickets for concurrent inference: Coordinate active/reservation budgets through the shared manager.

See concurrency.md for thread safety details.

Reference Documentation

• Arrays - Array creation, properties, indexing
• Operations - Math ops, broadcasting, reductions
• Transforms - grad, vmap, compile, eval
• Neural Networks - Layers and modules
• Optimizers - Training optimizers
• Custom Layers - Building custom modules
• Custom Kernels - Metal kernels with MLXFast
• Wired Memory - Ticket-based wired limit coordination
• Concurrency - Thread safety guide
• Deprecated - Migration guide