Customizing Vectors R

This skill covers type-stable operations and custom vector class design using the vctrs package.

Core Benefits

• Type stability - Predictable output types regardless of input values
• Size stability - Predictable output sizes from input sizes
• Consistent coercion rules - Single set of rules applied everywhere
• Robust class design - Proper S3 vector infrastructure

When to Use vctrs

Use vctrs when:

1. Building custom vector classes - Data frame compatibility, subsetting
2. Type-stable functions in packages - Guaranteed output types
3. Consistent coercion/casting - Explicit rules, predictable behavior
4. Size/length stability - Predictable sizing from inputs

Don't use vctrs when:

• Simple one-off analyses - Base R is sufficient
• No custom classes needed - Standard types work fine
• Performance critical + simple operations - Base R may be faster
• External API constraints - Must return base R types

vctrs vs Base R Decision Matrix

Building Custom Vector Classes

See custom-vector-class.md for the complete pattern:

• Constructor (low-level)
• Helper (user-facing)
• Format method

Type-Stable Functions

See type-stable-functions.md for:

• Guaranteed output types with vec_cast()
• Avoiding type-depends-on-data patterns

Coercion and Casting

See coercion-casting.md for:

• Explicit casting with clear rules
• Common type finding with vec_ptype_common()
• Self-coercion methods
• Cross-type coercion methods

Implementation Patterns

Coercion Methods

Define vec_ptype2.* methods for type compatibility:

vec_ptype2.pkg_percent.pkg_percent <- function(x, y, ...) new_percent()
vec_ptype2.pkg_percent.double <- function(x, y, ...) double()

Casting Methods

Define vec_cast.* methods for conversion:

vec_cast.pkg_percent.double <- function(x, to, ...) new_percent(x)
vec_cast.double.pkg_percent <- function(x, to, ...) vec_data(x)

See coercion-methods.md for complete examples.

Performance Considerations

When vctrs Adds Overhead

• Simple operations - vec_c(1, 2) vs c(1, 2)
• One-off scripts - Type safety less critical
• Small vectors - Overhead may outweigh benefits

When vctrs Improves Performance

• Package functions - Type stability prevents expensive re-computation
• Complex classes - Consistent behavior reduces debugging
• Data frame operations - Robust column type handling
• Repeated operations - Predictable types enable optimization

Package Development

Exports and Dependencies

# DESCRIPTION
Imports: vctrs

# NAMESPACE - Import what you need
importFrom(vctrs, vec_assert, new_vctr, vec_cast, vec_ptype_common)

Testing vctrs Classes

See testing-vctrs.md for:

• Type stability tests
• Coercion tests

Key Insight

vctrs is most valuable in package development where type safety, consistency, and extensibility matter more than raw speed for simple operations.

source: Sarah Johnson's gist https://gist.github.com/sj-io/3828d64d0969f2a0f05297e59e6c15ad