Compiling a CEL Program to Wasm

The ferricel build command compiles a CEL expression into a self-contained WebAssembly module.

The compiler removes all unused runtime functions from the final Wasm module. Beyond that, it performs no additional optimizations. For smaller or faster binaries, you can post-process the output with wasm-opt.

Basic Usage

Pass a CEL expression directly with --expression:

ferricel build --expression '1 + 1' --output result.wasm

Or provide the expression in a file with --expression-file:

ferricel build --expression-file my_program.cel --output result.wasm

The two flags are mutually exclusive. If --output is omitted, the module is written to final_cel_program.wasm in the current directory.

Host Extensions

Host extensions allow the compiled CEL expression to call functions implemented by the host at evaluation time. See the Host Extensions chapter for full documentation, including CLI flags, the Rust API, and builder chains.

Defining a CEL container

A CEL container acts as a namespace for type name resolution. When a container is set, unqualified type names are resolved relative to it first.

For example, with --container google.protobuf you can write Timestamp instead of google.protobuf.Timestamp in your CEL expression:

ferricel build \
  --expression 'Timestamp{seconds: 0}' \
  --container google.protobuf \
  --proto-descriptor descriptor.pb \
  --output result.wasm

Using Proto

To use Protocol Buffer message types in a CEL expression, provide a compiled descriptor set with --proto-descriptor. The flag can be repeated to merge multiple descriptor files:

ferricel build \
  --expression 'req.user_id != ""' \
  --proto-descriptor api.pb \
  --output result.wasm

Generate a descriptor set from your .proto files with protoc:

protoc --descriptor_set_out=api.pb --include_imports api.proto

Multiple descriptor files are merged automatically:

ferricel build \
  --expression 'a.field == b.field' \
  --proto-descriptor types_a.pb \
  --proto-descriptor types_b.pb \
  --output result.wasm