Cap URN - Go Implementation

Go implementation of Cap URN (Capability Uniform Resource Names), built on Tagged URN.

Features

• Required Direction Specifiers - in/out tags for input/output media types
• Media URN Validation - Validates direction spec values are valid Media URNs
• Special Pattern Values - * (must-have-any), ? (unspecified), ! (must-not-have)
• Graded Specificity - Exact values score higher than wildcards
• Cap Definitions - Full capability definitions with arguments, output, and metadata
• Cap Matrix - Registry for capability lookup and matching
• Cap Caller - Fluent API for invoking capabilities
• Schema Validation - JSON Schema validation for arguments and outputs

Installation

go get github.com/machinefabric/capdag-go

Quick Start

package main

import (
    "fmt"
    "log"

    "github.com/machinefabric/capdag-go"
)

func main() {
    // Parse a Cap URN
    cap, err := capdag.NewCapUrnFromString(`cap:in="media:binary";extract;out="media:object"`)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println("Input:", cap.GetInSpec())                          // "media:binary"
    fmt.Println("Output:", cap.GetOutSpec())                        // "media:object"
    fmt.Println("Has extract marker:", cap.HasMarkerTag("extract")) // true

    // Build a Cap URN
    built := capdag.NewCapUrnBuilder().
        InSpec("media:void").
        OutSpec("media:object").
        Marker("generate").
        Tag("target", "thumbnail").
        MustBuild()

    // Check matching
    pattern, _ := capdag.NewCapUrnFromString(`cap:in="media:binary";extract;out="media:object"`)
    if cap.Accepts(pattern) {
        fmt.Println("Cap matches pattern")
    }

    // Get specificity (graded scoring)
    fmt.Println("Specificity:", cap.Specificity())
}

Cap Definitions

// Create a full capability definition
capDef := &capdag.Cap{
    Urn:   cap,
    Title: "PDF Text Extractor",
    Args: []capdag.CapArg{
        {Name: "pages", Type: "string", Description: "Page range (e.g., '1-5')"},
    },
    Output: &capdag.CapOutput{
        Type:        "text",
        Description: "Extracted text content",
    },
}

Cap Matrix (Registry)

// Create a capability registry
matrix := capdag.NewCapMatrix()

// Register a capability with its handler
matrix.RegisterCapSet("my-cartridge", myHandler, []*capdag.Cap{capDef})

// Find matching capabilities
caps, err := matrix.FindCapSets(`cap:in="media:binary";extract;out=*`)

// Find the best match by specificity
host, cap, err := matrix.FindBestCapSet(requestUrn)

API Reference

CapUrn

Function/Method	Description
NewCapUrnFromString(s)	Parse Cap URN from string
NewCapUrnFromTags(tags)	Create from tag map (must include in/out)
GetInSpec()	Get input media URN
GetOutSpec()	Get output media URN
GetTag(key)	Get value for a tag key
WithTag(key, value)	Return new CapUrn with tag added/updated
WithInSpec(spec)	Return new CapUrn with changed input spec
WithOutSpec(spec)	Return new CapUrn with changed output spec
Accepts(request)	Check if Cap (as pattern) accepts a request
ConformsTo(pattern)	Check if Cap conforms to a pattern
Specificity()	Get graded specificity score
ToString()	Get canonical string representation

CapUrnBuilder

Method	Description
NewCapUrnBuilder()	Create a new builder
InSpec(spec)	Set input media URN (required)
OutSpec(spec)	Set output media URN (required)
Tag(key, value)	Add or update a tag (chainable)
Build()	Build the CapUrn (returns error if invalid)
MustBuild()	Build the CapUrn (panics if invalid)

Matching Semantics

Pattern	Instance Missing	Instance=v	Instance=x (x≠v)
(missing) or ?	Match	Match	Match
K=!	Match	No Match	No Match
K=*	No Match	Match	Match
K=v	No Match	Match	No Match

Graded Specificity

Value Type	Score
Exact value (K=v)	3
Must-have-any (K=*)	2
Must-not-have (K=!)	1
Unspecified (K=?) or missing	0

Error Codes

Code	Constant	Description
10	ErrorMissingInSpec	Missing required in tag
11	ErrorMissingOutSpec	Missing required out tag
12	ErrorInvalidMediaUrn	Invalid Media URN in direction spec

For base Tagged URN error codes, see Tagged URN documentation.

Testing

go test -v ./...

Cross-Language Compatibility

This Go implementation produces identical results to:

• Rust reference implementation
• JavaScript implementation
• Objective-C implementation

All implementations follow the same rules. See:

• Cap URN RULES.md - Cap-specific rules
• Tagged URN RULES.md - Base format rules