Docs 2.4.4 · 5 min read

Building AI with Apsi

Apsi does not force a ready-made architecture on you. Every network is an ordinary function that takes a "parameter tree" (record / list / struct), and grad differentiates with respect to every parameter in it. Whatever architecture you invent, you can write it down directly.

1. Build any architecture with use layers

Every layer is a pair of pure functions — init(seed) creates its parameters, layer(params, x) applies it — and layers compose into graphs. Models are ordinary values: grad, optimizers, save_params and fit work on them. This section is an overview; LAYERS.md is the complete reference (every layer with its shapes and parameters, error messages, recipes).

use layers
use nn

let model = sequential([linear(2, 64), activation("gelu"), dropout_layer(0.1), linear(64, 3)])
var params = model.init(seed=0)
let logits = model(params, X)                       # model(params, X, train=true) turns dropout on
let r = fit(model, params, X, y, loss="cross_entropy", opt=adam(lr=0.01), epochs=50, batch_size=64)
summary(model, r.params)                            # layer table with parameter counts

Any graph of layers — each node is [name, layer, input(s)]:

let net = dag([
    ["wide", linear(2, 3), "x"],
    ["deep", mlp([2, 32, 32, 3]), "x"],
    ["both", add(), ["wide", "deep"]],
], inputs=["x"], output="both")

Graph neural networks — pass the graph when applying or fitting; mask limits the loss to labelled nodes:

let g = graph(edges=[[0, 1], [1, 2], [2, 0]], n=3, undirected=true)
let gnn = gat_net(8, 16, classes, heads=4)
let r = fit(gnn, gnn.init(seed=0), features, labels, loss="cross_entropy", graph=g, mask=train_mask, epochs=200)

Model zoo — mlp(sizes), resnet_mlp(in, out, width, blocks), cnn(channels, classes, image), lstm_classifier(in, hidden, classes), transformer_lm(vocab, d, heads, layers, context) (GPT-style), transformer_classifier(vocab, classes, …), autoencoder(in, latent), vae(in, latent) with vae_loss, gcn_net, gat_net, gin_net (graph classification), siamese(encoder, combine, head).

Training — fit(model, params, X, y, loss="mse"|"cross_entropy"|"mae"|"bce"|"vae"|fn, opt, epochs, batch_size, clip, schedule, graph, mask, val=(Xv, yv)) returns record(params, history, val_history); predict, summary, count_params. Full example: examples/28_any_architecture.apsi.

On GPUs with PyTorch — the same model, trained by PyTorch on CUDA / ROCm / Apple MPS / Intel XPU with mixed precision and torch.compile; the result is a normal Apsi parameter tree again (details: BACKENDS.md):

let r = fit(model, params, X, y, loss="cross_entropy", epochs=20, batch_size=256,
            backend="torch", device="cuda", precision="bf16", compile=true)     # run with --allow-python
let m = torch_module(model, r.params)        # a real torch.nn.Module: state_dict, DDP, ONNX export
check_torch()                                # every layer compared with NumPy (outputs and gradients)

2. Design your own, function by function

use nn

# a home-made architecture: gated residual blocks
fn block(p, x):
    let gate = sigmoid(x @ p.g.W + p.g.b)
    let h = gelu(layer_norm(x) @ p.f.W + p.f.b)
    return x + gate * h

fn model(p, x):
    var h = x @ p.inp.W
    for layer in p.blocks:
        h = block(layer, h)
    return h @ p.out.W

let params = record(inp=dense(16, 64), blocks=[record(g=dense(64, 64), f=dense(64, 64)) for _ in 0..4],
                    out=dense(64, 1))
let loss, grads = value_and_grad(fn(p) => mse(model(p, X), y))(params)

Building blocks:

3. Use less RAM / VRAM

4. Use pretrained models from other ecosystems

Install the library with pip as usual and call it from Apsi (run with --allow-python):

use models
let classify = hf_pipeline("sentiment-analysis")          # pip install transformers torch
print(classify("Apsi is great"))

let net = onnx_model("resnet50.onnx")                       # pip install onnxruntime(-gpu)
let logits = net(image_batch)                               # picks TensorRT / CUDA / DirectML / CPU for you

let weights = torch_load("checkpoint.pt")                   # pip install torch (safe weights_only mode)
let hf_weights = load_safetensors("model.safetensors", dtype=int8)   # no extra library needed

Existing PyTorch/numpy code can be translated with apsi convert model.py: nn.Module models, losses, optimizers, data loaders and training loops become native Apsi that runs without Python or PyTorch, with the same outputs and gradients as PyTorch for the same weights. load_torch_weights(model, params, state_dict) imports pretrained weights (see PYTHON.md). Any other Python library: pyimport("module"), then to_torch / from_torch to move tensors back and forth (after use layers, torch_module(model, params) turns a whole Apsi model into a torch.nn.Module). apsi pkg install ai installs ONNX Runtime, PyTorch and Transformers into Apsi's own runtime.

5. Serve it as an API

use web
let app = server()
@app.post("/predict")
fn predict(req) = {"y": model(params, tensor(req.json()["x"]))}
app.run(port=8080)

Complete examples: examples/23_web_api.apsi, 26_model_serving.apsi and 27_pretrained_models.apsi. Server security is covered in SERVER.md.

6. Save and share models

save_model writes an Apsi checkpoint (.apsim): a standard safetensors file that also keeps units, compressed tensors and the parameter tree, checks a SHA-256 when loading, can be signed, and splits huge models into shards. save_gguf writes GGUF for llama.cpp, Ollama and LM Studio, with Q8_0 and Q4_0 bit-identical to ggml. apsi model convert converts between safetensors, GGUF and .apsim. Full reference: MODEL-FILES.md.

use models
save_model("ckpt/model.apsim", params, metadata=record(step=step), sign=true)
let params = load_model("ckpt/model.apsim", key=true)
save_gguf("model-q8.gguf", params, dtype="q8_0")

Model file safety

• load_safetensors never runs code from the file (unlike pickle) and checks that every offset stays inside the file
• load_model and load_gguf check every header size and offset against the file before mapping it, and load_model checks the SHA-256 (and the signature when you give a key)
• torch_load uses weights_only=True, so code embedded in a .pt file is not executed
• the on-disk cache is signed with HMAC

Found a mistake? Report it · Markdown source