Fluid Script: Simple .NET server side scripting with browser breakpoints

FluidScript is a small, statically checked scripting language for .NET applications. It is designed for products that need approachable, user-authored automation or business rules without granting scripts ambient access to the file system, network, processes, or arbitrary CLR reflection.

Scripts compile to verified, deterministic P-code and run on a purpose-built virtual machine. Your application explicitly chooses the functions, data, and CLR types a script may use.

• Friendly authoring model — readable declarations, functions, types, arrays, dictionaries, string interpolation, JSON, error handling, and familiarif ,while ,for , andswitch control flow.
• Early, useful feedback — the compiler reports source-mapped diagnostics with stable codes before a script runs; runtime faults retain their source location and script function context.
• Capability-based embedding — the host registers only the operations and CLR types it intends to expose. There is no implicit I/O, process, network, or reflection capability.
• Predictable execution — a verified stack VM with configurable instruction limits, a bounded call depth, deterministic P-code serialization, and explicit values flowing across the host boundary.
• Debuggable scripts — debug P-code preserves source spans. The VM can stop on source lines and create a portable, self-contained checkpoint that another process can validate and resume.

function greeting(name: string): string
    return "Hello, {name}!"
end

dim names = ["Ada", "Grace", "Lin"]
for index = 0 to 2
    print(greeting(names[index]))
end

The standard library is available automatically, including String, RegExp, Number, and Math:

dim amount = Number.parseFloat("19.95")
dim tax = Math.round(amount * 0.08 * 100) / 100
print("Total tax: {tax}")

FluidScript fits well when an application needs controlled extensibility, for example:

• configurable business rules, calculations, and validations;
• workflow steps and domain-specific automation;
• user-authored formulas and transformations over application-supplied data;
• scripted behavior in a .NET desktop, service, or web application; and
• an editor/playground experience with compiler diagnostics and source-level breakpoints.

It is not a replacement for unrestricted application code. The host owns security policy, data access, and every external capability exposed to a script. Give untrusted scripts only narrowly scoped functions and a suitable instruction budget.

FluidScript targets .NET 10. Build and run the test suite from the repository root:

dotnet test FluidScript.sln --configuration Release --no-restore

To try the included Monaco-based browser playground:

dotnet run --project fluid-script-monaco/FluidScript.Monaco.csproj

Open the local URL printed by ASP.NET Core (normally http://localhost:5000 or https://localhost:5001). The playground supports editing, syntax highlighting, diagnostics, execution, breakpoints, detached debug checkpoints, and JSON-native variable edits. It is a development sample, not a production-ready multi-user execution service.

The release helpers increment the patch component in fluid-script/FluidScript.csproj, create a Release package in artifacts/nuget/, and persist the version only after packaging succeeds.

On Windows:

pack-nuget.bat

On macOS or another zsh host:

./pack-nuget.sh

• User manual — learn the language, values, functions, flow control, JSON, imports, and the standard libraries.
• Integration guide — add FluidScript to a .NET project, execute and debug scripts, register application libraries/CLR types, persist P-code, and provide a module resolver.
• Language contract — the normative language semantics and diagnostic behavior.
• P-code format — serialization and detached-debugging contracts.
• Monaco sample — details of the browser playground and its HTTP protocol.

import is intentionally host-resolved: an application supplies an IFluidModuleResolver that maps a script-visible name to source. Imports currently validate and compile dependencies, including missing-module and cycle detection. They do not yet execute modules, export symbols, or create script namespaces. Use explicit host capabilities for shared behavior until the staged module-linking features are available.

fluid-script/         Compiler, P-code model, virtual machine, and host API
fluid-script.test/    MSTest coverage for language and runtime behavior
fluid-script-monaco/  ASP.NET Core Monaco playground
docs/                 User, integration, language, P-code, and architecture docs