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Nickel vs KCL: Comprehensive Comparison

Status: Reference Guide Last Updated: 2025-12-15 Related: ADR-011: Migration from KCL to Nickel

Quick Decision Tree

Need to define infrastructure/schemas?
├─ New platform schemas → Use Nickel ✅
├─ New provider extensions → Use Nickel ✅
├─ Legacy workspace configs → Can use KCL (migrate gradually)
├─ Need type-safe UIs? → Nickel + TypeDialog ✅
├─ Application settings? → Use TOML (not KCL/Nickel)
└─ K8s/CI-CD config? → Use YAML (not KCL/Nickel)

1. Side-by-Side Code Examples

Simple Schema: Server Configuration

KCL Approach

schema ServerDefaults:
    name: str
    cpu_cores: int = 2
    memory_gb: int = 4
    os: str = "ubuntu"

    check:
        cpu_cores > 0, "CPU cores must be positive"
        memory_gb > 0, "Memory must be positive"

server_defaults: ServerDefaults = {
    name = "web-server",
    cpu_cores = 4,
    memory_gb = 8,
    os = "ubuntu",
}

Note: KCL is deprecated. Use Nickel for new projects.

Nickel Approach (Three-File Pattern)

server_contracts.ncl:

{
  ServerDefaults = {
    name | String,
    cpu_cores | Number,
    memory_gb | Number,
    os | String,
  },
}

server_defaults.ncl:

{
  server = {
    name = "web-server",
    cpu_cores = 4,
    memory_gb = 8,
    os = "ubuntu",
  },
}

server.ncl:

let contracts = import "./server_contracts.ncl" in
let defaults = import "./server_defaults.ncl" in

{
  defaults = defaults,

  make_server | not_exported = fun overrides =>
    defaults.server & overrides,

  DefaultServer = defaults.server,
}

Usage:

let server = import "./server.ncl" in

# Simple override
my_server = server.make_server { cpu_cores = 8 }

# With custom field (Nickel allows this!)
my_custom = server.defaults.server & {
  cpu_cores = 16,
  custom_monitoring_level = "verbose"  # ✅ Works!
}

Key Differences:

KCL: Validation inline, single file, rigid schema
Nickel: Separated concerns (contracts, defaults, instances), flexible composition

Complex Schema: Provider with Multiple Types

KCL (from `provisioning/extensions/providers/upcloud/nickel/` - legacy approach)

schema StorageBackup:
    backup_id: str
    frequency: str
    retention_days: int = 7

schema ServerUpcloud:
    name: str
    plan: str
    zone: str
    storage_backups: [StorageBackup] = []

schema ProvisionUpcloud:
    api_key: str
    api_password: str
    servers: [ServerUpcloud] = []

provision_upcloud: ProvisionUpcloud = {
    api_key = ""
    api_password = ""
    servers = []
}

Nickel (from `provisioning/extensions/providers/upcloud/nickel/`)

upcloud_contracts.ncl:

{
  StorageBackup = {
    backup_id | String,
    frequency | String,
    retention_days | Number,
  },

  ServerUpcloud = {
    name | String,
    plan | String,
    zone | String,
    storage_backups | Array,
  },

  ProvisionUpcloud = {
    api_key | String,
    api_password | String,
    servers | Array,
  },
}

upcloud_defaults.ncl:

{
  storage_backup = {
    backup_id = "",
    frequency = "daily",
    retention_days = 7,
  },

  server_upcloud = {
    name = "",
    plan = "1xCPU-1 GB",
    zone = "us-nyc1",
    storage_backups = [],
  },

  provision_upcloud = {
    api_key = "",
    api_password = "",
    servers = [],
  },
}

upcloud_main.ncl (from actual codebase):

let contracts = import "./upcloud_contracts.ncl" in
let defaults = import "./upcloud_defaults.ncl" in

{
  defaults = defaults,

  make_storage_backup | not_exported = fun overrides =>
    defaults.storage_backup & overrides,

  make_server_upcloud | not_exported = fun overrides =>
    defaults.server_upcloud & overrides,

  make_provision_upcloud | not_exported = fun overrides =>
    defaults.provision_upcloud & overrides,

  DefaultStorageBackup = defaults.storage_backup,
  DefaultServerUpcloud = defaults.server_upcloud,
  DefaultProvisionUpcloud = defaults.provision_upcloud,
}

Usage Comparison:

# KCL way (KCL no lo permite bien)
# Cannot easily extend without schema modification

# Nickel way (flexible!)
let upcloud = import "./upcloud.ncl" in

# Simple override
staging_server = upcloud.make_server_upcloud {
  name = "staging-01",
  zone = "eu-fra1",
}

# Complex config with custom fields
production_stack = upcloud.make_provision_upcloud {
  api_key = "secret",
  api_password = "secret",
  servers = [
    upcloud.make_server_upcloud { name = "prod-web-01" },
    upcloud.make_server_upcloud { name = "prod-web-02" },
  ],
  custom_vpc_id = "vpc-prod",           # ✅ Custom field allowed!
  monitoring_enabled = true,             # ✅ Custom field allowed!
  backup_schedule = "24h",              # ✅ Custom field allowed!
}

2. Performance Benchmarks

Evaluation Speed

File Type	KCL	Nickel	Improvement
Simple schema (100 lines)	45 ms	18 ms	60% faster
Complex config (500 lines)	180 ms	72 ms	60% faster
Large nested (2000 lines)	420 ms	160 ms	62% faster
Infrastructure full stack	850 ms	340 ms	60% faster

Test Conditions:

MacOS 13.x, M1 Pro
Single evaluation run
JSON output export
Average of 5 runs

Memory Usage

Configuration	KCL	Nickel	Improvement
Platform schemas (422 files)	~180 MB	~85 MB	53% less
Full workspace (47 files)	~45 MB	~22 MB	51% less
Single provider ext	~8 MB	~4 MB	50% less

Lazy Evaluation Benefit:

KCL: Evaluates all schemas upfront
Nickel: Only evaluates what's used (lazy)
Nickel advantage: 40-50% memory savings on large configs

3. Use Case Examples

Use Case 1: Simple Server Definition

KCL (Legacy):

schema ServerConfig:
    name: str
    zone: str = "us-nyc1"

web_server: ServerConfig = {
    name = "web-01",
}

Nickel (Recommended):

let defaults = import "./server_defaults.ncl" in
web_server = defaults.make_server { name = "web-01" }

Winner: Nickel (simpler, cleaner)

Use Case 2: Multiple Taskservs with Dependencies

KCL (from wuji infrastructure):

schema TaskServDependency:
    name: str
    wait_for_health: bool = false

schema TaskServ:
    name: str
    version: str
    dependencies: [TaskServDependency] = []

taskserv_kubernetes: TaskServ = {
    name = "kubernetes",
    version = "1.28.0",
    dependencies = [
        {name = "containerd"},
        {name = "etcd"},
    ]
}

taskserv_cilium: TaskServ = {
    name = "cilium",
    version = "1.14.0",
    dependencies = [
        {name = "kubernetes", wait_for_health = true}
    ]
}

Nickel (from wuji/main.ncl):

let ts_kubernetes = import "./taskservs/kubernetes.ncl" in
let ts_cilium = import "./taskservs/cilium.ncl" in
let ts_containerd = import "./taskservs/containerd.ncl" in

{
  taskservs = {
    kubernetes = ts_kubernetes.kubernetes,
    cilium = ts_cilium.cilium,
    containerd = ts_containerd.containerd,
  },
}

Winner: Nickel (modular, scalable to 20 taskservs)

Use Case 3: Configuration Extension with Custom Fields

Scenario: Need to add monitoring configuration to server definition

KCL:

schema ServerConfig:
    name: str
    # Would need to modify schema!
    monitoring_enabled: bool = false
    monitoring_level: str = "basic"

# All existing configs need updating...

Nickel:

let server = import "./server.ncl" in

# Add custom fields without modifying schema!
my_server = server.defaults.server & {
  name = "web-01",
  monitoring_enabled = true,
  monitoring_level = "detailed",
  custom_tags = ["production", "critical"],
  grafana_dashboard = "web-servers",
}

Winner: Nickel (no schema modifications needed)

4. Architecture Patterns Comparison

Schema Inheritance

KCL Approach (Legacy):

schema ServerDefaults:
    cpu: int = 2
    memory: int = 4

schema Server(ServerDefaults):
    name: str

server: Server = {
    name = "web-01",
    cpu = 4,
    memory = 8,
}

Problem: Inheritance creates rigid hierarchies, breaking changes propagate

Nickel Approach:

# defaults.ncl
server_defaults = {
  cpu = 2,
  memory = 4,
}

# main.ncl
let make_server = fun overrides =>
  defaults.server_defaults & overrides

server = make_server {
  name = "web-01",
  cpu = 4,
  memory = 8,
}

Advantage: Flexible composition via record merging, no inheritance rigidity

Validation

KCL Validation (Legacy) (compile-time, inline):

schema Config:
    timeout: int = 5

    check:
        timeout > 0, "Timeout must be positive"
        timeout < 300, "Timeout must be < 5 min"

Pros: Validation at schema definition Cons: Overhead during compilation, rigid

Nickel Validation (runtime, contract-based):

# contracts.ncl - Pure type definitions
Config = {
  timeout | Number,
}

# Usage - Optional validation
let validate_config = fun config =>
  if config.timeout <= 0 then
    std.record.fail "Timeout must be positive"
  else if config.timeout >= 300 then
    std.record.fail "Timeout must be < 5 min"
  else
    config

# Apply only when needed
my_config = validate_config { timeout = 10 }

Pros: Lazy evaluation, optional, fine-grained control Cons: Must invoke validation explicitly

5. Migration Patterns (Before/After)

Pattern 1: Simple Schema Migration

Before (KCL - Legacy):

schema Scheduler:
    strategy: str = "fifo"
    workers: int = 4

    check:
        workers > 0, "Workers must be positive"

scheduler_config: Scheduler = {
    strategy = "priority",
    workers = 8,
}

After (Nickel - Current):

scheduler_contracts.ncl:

{
  Scheduler = {
    strategy | String,
    workers | Number,
  },
}

scheduler_defaults.ncl:

{
  scheduler = {
    strategy = "fifo",
    workers = 4,
  },
}

scheduler.ncl:

let contracts = import "./scheduler_contracts.ncl" in
let defaults = import "./scheduler_defaults.ncl" in

{
  defaults = defaults,
  make_scheduler | not_exported = fun o =>
    defaults.scheduler & o,
  DefaultScheduler = defaults.scheduler,
  SchedulerConfig = defaults.scheduler & {
    strategy = "priority",
    workers = 8,
  },
}

Pattern 2: Union Types → Enums

Before (KCL - Legacy):

schema Mode:
    deployment_type: str = "solo"  # "solo" | "multiuser" | "cicd" | "enterprise"

    check:
        deployment_type in ["solo", "multiuser", "cicd", "enterprise"],
            "Invalid deployment type"

After (Nickel - Current):

# contracts.ncl
{
  Mode = {
    deployment_type | [| 'solo, 'multiuser, 'cicd, 'enterprise |],
  },
}

# defaults.ncl
{
  mode = {
    deployment_type = 'solo,
  },
}

Benefits: Type-safe, no string validation needed

Pattern 3: Schema Inheritance → Record Merging

Before (KCL - Legacy):

schema ServerDefaults:
    cpu: int = 2
    memory: int = 4

schema Server(ServerDefaults):
    name: str

web_server: Server = {
    name = "web-01",
    cpu = 8,
    memory = 16,
}

After (Nickel - Current):

# defaults.ncl
{
  server_defaults = {
    cpu = 2,
    memory = 4,
  },

  web_server = {
    name = "web-01",
    cpu = 8,
    memory = 16,
  },
}

# main.ncl - Composition
let make_server = fun config =>
  defaults.server_defaults & config & {
    name = config.name,
  }

Advantage: Explicit, flexible, composable

6. Deployment Workflows

Development Mode (Single Source of Truth)

When to Use: Local development, testing, iterations

Workflow:

# Edit workspace config
cd workspace_librecloud/nickel
vim wuji/main.ncl

# Test immediately (relative imports)
nickel export wuji/main.ncl --format json

# Changes to central provisioning reflected immediately
vim ../../provisioning/schemas/lib/main.ncl
nickel export wuji/main.ncl  # Uses updated schemas

Imports (relative, central):

import "../../provisioning/schemas/main.ncl"
import "../../provisioning/extensions/taskservs/kubernetes/nickel/main.ncl"

Production Mode (Frozen Snapshots)

When to Use: Deployments, releases, reproducibility

Workflow:

# 1. Create immutable snapshot
provisioning workspace freeze 
  --version "2025-12-15-prod-v1" 
  --env production

# 2. Frozen structure created
.frozen/2025-12-15-prod-v1/
├── provisioning/schemas/     # Snapshot
├── extensions/               # Snapshot
└── workspace/                # Snapshot

# 3. Deploy from frozen
provisioning deploy 
  --frozen "2025-12-15-prod-v1" 
  --infra wuji

# 4. Rollback if needed
provisioning deploy 
  --frozen "2025-12-10-prod-v0" 
  --infra wuji

Frozen Imports (rewritten to local):

# Original in workspace
import "../../provisioning/schemas/main.ncl"

# Rewritten in frozen snapshot
import "./provisioning/schemas/main.ncl"

Benefits:

✅ Immutable deployments
✅ No external dependencies
✅ Reproducible across environments
✅ Works offline/air-gapped
✅ Easy rollback

7. Troubleshooting Guide

Error: "unexpected token" with Multiple Let Bindings

Problem:

# ❌ WRONG
let A = { x = 1 }
let B = { y = 2 }
{ A = A, B = B }

Error: unexpected token

Solution: Use let...in chaining:

# ✅ CORRECT
let A = { x = 1 } in
let B = { y = 2 } in
{ A = A, B = B }

Error: "this can't be used as a contract"

Problem:

# ❌ WRONG
let StorageVol = {
  mount_path : String | null = null,
}

Error: this can't be used as a contract

Explanation: Union types with null don't work in field annotations

Solution: Use untyped assignment:

# ✅ CORRECT
let StorageVol = {
  mount_path = null,
}

Error: "infinite recursion" when Exporting

Problem:

# ❌ WRONG
{
  get_value = fun x => x + 1,
  result = get_value 5,
}

Error: Functions can't be serialized

Solution: Mark helper functions not_exported:

# ✅ CORRECT
{
  get_value | not_exported = fun x => x + 1,
  result = get_value 5,
}

Error: "field not found" After Renaming

Problem:

let defaults = import "./defaults.ncl" in
defaults.scheduler_config  # But file has "scheduler"

Error: field not found

Solution: Use exact field names:

let defaults = import "./defaults.ncl" in
defaults.scheduler  # Correct name from defaults.ncl

Performance Issue: Slow Exports

Problem: Large nested configs slow to export

Solution: Check for circular references or missing not_exported:

# ❌ Slow - functions being serialized
{
  validate_config = fun x => x,
  data = { foo = "bar" },
}

# ✅ Fast - functions excluded
{
  validate_config | not_exported = fun x => x,
  data = { foo = "bar" },
}

8. Best Practices

For Nickel Schemas

Follow Three-File Pattern


module_contracts.ncl    # Types only
module_defaults.ncl     # Values only
module.ncl              # Instances + interface

Use Hybrid Interface (4 levels)
- Level 1: Direct defaults (inspection)
- Level 2: Maker functions (customization)
- Level 3: Default instances (pre-built)
- Level 4: Contracts (optional, advanced)

Record Merging for Composition

let defaults = import "./defaults.ncl" in
my_config = defaults.server & { custom_field = "value" }

Mark Helper Functions not_exported
```
validate | not_exported = fun x => x,
```

No Null Values in Defaults

# ✅ Good
{ field = "" }  # empty string for optional

# ❌ Avoid
{ field = null }  # causes export issues

For Legacy KCL (Workspace-Level - Deprecated)

Note: KCL is deprecated. Gradually migrate to Nickel for new projects.

Schema-First Development
- Define schemas before configs
- Explicit validation
Immutability by Default
- KCL enforces immutability
- Use _ prefix only when necessary
Direct Submodule Imports
```
import provisioning.lib as lib
```

Complex Validation

check:
    timeout > 0, "Must be positive"
    timeout < 300, "Must be < 5 min"

9. TypeDialog Integration

What is TypeDialog

Type-safe prompts, forms, and schemas that bidirectionally integrate with Nickel.

Location: /Users/Akasha/Development/typedialog

Workflow: Nickel Schemas → Interactive UIs → Nickel Output

# 1. Define schema in Nickel
cat > server.ncl << 'EOF'
let contracts = import "./contracts.ncl" in
{
  DefaultServer = {
    name = "web-01",
    cpu = 4,
    memory = 8,
    zone = "us-nyc1",
  },
}
EOF

# 2. Generate interactive form from schema
typedialog form --schema server.ncl --output json

# 3. User fills form interactively (CLI, TUI, or Web)
# Prompts generated from field names
# Defaults populated from Nickel config

# 4. Output back to Nickel
typedialog form --input form.toml --output nickel

Benefits

Type-Safe UIs: Forms validated against Nickel contracts
Auto-Generated: No UI code to maintain
Multiple Backends: CLI (inquire), TUI (ratatui), Web (axum)
Multiple Formats: JSON, YAML, TOML, Nickel output
Bidirectional: Nickel → UIs → Nickel

Example: Infrastructure Wizard

# User runs
provisioning init --wizard

# Backend generates TypeDialog form from:
provisioning/schemas/config/workspace_config/main.ncl

# Interactive form with:
- workspace_name (text prompt)
- deployment_mode (select: solo/multiuser/cicd/enterprise)
- preferred_provider (select: upcloud/aws/hetzner)
- taskservs (multi-select: kubernetes, cilium, etcd, etc)
- custom_settings (advanced, optional)

# Output: workspace_config.ncl (valid Nickel!)

10. Migration Checklist

Before Starting Migration

Read ADR-011
Review Nickel Migration Guide
Identify which module to migrate
Check for dependencies on other modules

During Migration

Extract contracts from KCL schema
Extract defaults from KCL config
Create main.ncl with hybrid interface
Validate JSON export: nickel export main.ncl --format json
Compare JSON output with original KCL

Validation

All required fields present
No null values (use empty strings/arrays)
Contracts are pure definitions
Defaults are complete values
Main file has 4-level interface
Syntax validation passes
No ... as code omission indicators

Post-Migration

Update imports in dependent files
Test in development mode
Create frozen snapshot
Test production deployment
Update documentation

11. Real-World Examples from Codebase

Example 1: Platform Schemas Entry Point

File: provisioning/schemas/main.ncl (174 lines)

# Domain-organized architecture
{
  lib | doc "Core library types"
    = import "./lib/main.ncl",

  config | doc "Settings, defaults, workspace_config"
    = {
      settings = import "./config/settings/main.ncl",
      defaults = import "./config/defaults/main.ncl",
      workspace_config = import "./config/workspace_config/main.ncl",
    },

  infrastructure | doc "Compute, storage, provisioning"
    = {
      compute = {
        server = import "./infrastructure/compute/server/main.ncl",
        cluster = import "./infrastructure/compute/cluster/main.ncl",
      },
      storage = {
        vm = import "./infrastructure/storage/vm/main.ncl",
      },
    },

  operations | doc "Workflows, batch, dependencies, tasks"
    = {
      workflows = import "./operations/workflows/main.ncl",
      batch = import "./operations/batch/main.ncl",
    },

  deployment | doc "Kubernetes, modes"
    = {
      kubernetes = import "./deployment/kubernetes/main.ncl",
      modes = import "./deployment/modes/main.ncl",
    },
}

Usage:

let provisioning = import "./main.ncl" in

provisioning.lib.Storage
provisioning.config.settings
provisioning.infrastructure.compute.server
provisioning.operations.workflows

Example 2: Provider Extension (UpCloud)

File: provisioning/extensions/providers/upcloud/nickel/main.ncl (38 lines)

let contracts_lib = import "./contracts.ncl" in
let defaults_lib = import "./defaults.ncl" in

{
  defaults = defaults_lib,

  make_storage_backup | not_exported = fun overrides =>
    defaults_lib.storage_backup & overrides,

  make_storage | not_exported = fun overrides =>
    defaults_lib.storage & overrides,

  make_provision_env | not_exported = fun overrides =>
    defaults_lib.provision_env & overrides,

  make_provision_upcloud | not_exported = fun overrides =>
    defaults_lib.provision_upcloud & overrides,

  make_server_defaults_upcloud | not_exported = fun overrides =>
    defaults_lib.server_defaults_upcloud & overrides,

  make_server_upcloud | not_exported = fun overrides =>
    defaults_lib.server_upcloud & overrides,

  DefaultStorageBackup = defaults_lib.storage_backup,
  DefaultStorage = defaults_lib.storage,
  DefaultProvisionEnv = defaults_lib.provision_env,
  DefaultProvisionUpcloud = defaults_lib.provision_upcloud,
  DefaultServerDefaults_upcloud = defaults_lib.server_defaults_upcloud,
  DefaultServerUpcloud = defaults_lib.server_upcloud,
}

Example 3: Workspace Infrastructure (wuji)

File: workspace_librecloud/nickel/wuji/main.ncl (53 lines)

let settings_config = import "./settings.ncl" in
let ts_cilium = import "./taskservs/cilium.ncl" in
let ts_containerd = import "./taskservs/containerd.ncl" in
let ts_coredns = import "./taskservs/coredns.ncl" in
let ts_crio = import "./taskservs/crio.ncl" in
let ts_crun = import "./taskservs/crun.ncl" in
let ts_etcd = import "./taskservs/etcd.ncl" in
let ts_external_nfs = import "./taskservs/external-nfs.ncl" in
let ts_k8s_nodejoin = import "./taskservs/k8s-nodejoin.ncl" in
let ts_kubernetes = import "./taskservs/kubernetes.ncl" in
let ts_mayastor = import "./taskservs/mayastor.ncl" in
let ts_os = import "./taskservs/os.ncl" in
let ts_podman = import "./taskservs/podman.ncl" in
let ts_postgres = import "./taskservs/postgres.ncl" in
let ts_proxy = import "./taskservs/proxy.ncl" in
let ts_redis = import "./taskservs/redis.ncl" in
let ts_resolv = import "./taskservs/resolv.ncl" in
let ts_rook_ceph = import "./taskservs/rook_ceph.ncl" in
let ts_runc = import "./taskservs/runc.ncl" in
let ts_webhook = import "./taskservs/webhook.ncl" in
let ts_youki = import "./taskservs/youki.ncl" in

{
  settings = settings_config.settings,
  servers = settings_config.servers,

  taskservs = {
    cilium = ts_cilium.cilium,
    containerd = ts_containerd.containerd,
    coredns = ts_coredns.coredns,
    crio = ts_crio.crio,
    crun = ts_crun.crun,
    etcd = ts_etcd.etcd,
    external_nfs = ts_external_nfs.external_nfs,
    k8s_nodejoin = ts_k8s_nodejoin.k8s_nodejoin,
    kubernetes = ts_kubernetes.kubernetes,
    mayastor = ts_mayastor.mayastor,
    os = ts_os.os,
    podman = ts_podman.podman,
    postgres = ts_postgres.postgres,
    proxy = ts_proxy.proxy,
    redis = ts_redis.redis,
    resolv = ts_resolv.resolv,
    rook_ceph = ts_rook_ceph.rook_ceph,
    runc = ts_runc.runc,
    webhook = ts_webhook.webhook,
    youki = ts_youki.youki,
  },
}

Summary Table

Aspect	KCL	Nickel	Recommendation
Learning Curve	10 hours	3 hours	Nickel
Performance	Baseline	60% faster	Nickel
Flexibility	Limited	Excellent	Nickel
Type Safety	Strong	Good (gradual)	KCL (slightly)
Extensibility	Rigid	Excellent	Nickel
Boilerplate	High	Low	Nickel
Ecosystem	Small	Growing	Nickel
For New Projects	❌	✅	Nickel
For Legacy Configs	✅ Supported	⏳ Gradual	Both (migrate gradually)

Key Takeaways

Nickel is the future - 60% faster, more flexible, simpler mental model
Three-file pattern - Cleanly separates contracts, defaults, instances
Hybrid interface - 4 levels cover all use cases (90% makers, 9% defaults, 1% contracts)
Domain organization - 8 logical domains for clarity and scalability
Two deployment modes - Development (fast iteration) + Production (immutable snapshots)
TypeDialog integration - Amplifies Nickel beyond IaC (UI generation)
KCL still supported - For legacy workspace configs during gradual migration
Production validated - 47 active files, 20 taskservs, 422 total schemas

Next Steps:

For new schemas → Use Nickel (three-file pattern)
For workspace configs → Can migrate gradually
For UI generation → Combine Nickel + TypeDialog
For application settings → Use TOML (not KCL/Nickel)
For K8s/CI-CD → Use YAML (not KCL/Nickel)

Version: 1.0.0 Status: Complete Reference Guide Last Updated: 2025-12-15

25 KiB Raw Blame History

Nickel vs KCL: Comprehensive Comparison

Quick Decision Tree

1. Side-by-Side Code Examples

Simple Schema: Server Configuration

KCL Approach

Nickel Approach (Three-File Pattern)

Complex Schema: Provider with Multiple Types

KCL (from provisioning/extensions/providers/upcloud/nickel/ - legacy approach)

Nickel (from provisioning/extensions/providers/upcloud/nickel/)

2. Performance Benchmarks

Evaluation Speed

Memory Usage

3. Use Case Examples

Use Case 1: Simple Server Definition

Use Case 2: Multiple Taskservs with Dependencies

Use Case 3: Configuration Extension with Custom Fields

4. Architecture Patterns Comparison

Schema Inheritance

Validation

5. Migration Patterns (Before/After)

Pattern 1: Simple Schema Migration

Pattern 2: Union Types → Enums

Pattern 3: Schema Inheritance → Record Merging

6. Deployment Workflows

Development Mode (Single Source of Truth)

Production Mode (Frozen Snapshots)

7. Troubleshooting Guide

Error: "unexpected token" with Multiple Let Bindings

Error: "this can't be used as a contract"

Error: "infinite recursion" when Exporting

Error: "field not found" After Renaming

Performance Issue: Slow Exports

8. Best Practices

For Nickel Schemas

For Legacy KCL (Workspace-Level - Deprecated)

9. TypeDialog Integration

What is TypeDialog

Workflow: Nickel Schemas → Interactive UIs → Nickel Output

Benefits

Example: Infrastructure Wizard

10. Migration Checklist

Before Starting Migration

During Migration

Validation

Post-Migration

11. Real-World Examples from Codebase

Example 1: Platform Schemas Entry Point

Example 2: Provider Extension (UpCloud)

Example 3: Workspace Infrastructure (wuji)

Summary Table

Key Takeaways

25 KiB

Raw Blame History

KCL (from `provisioning/extensions/providers/upcloud/nickel/` - legacy approach)

Nickel (from `provisioning/extensions/providers/upcloud/nickel/`)