Ruby 3.4 Adds Array#fetch_values for Safe Multi-Index Access
· 4 min read
Accessing multiple array elements by index often requires repetitive nil checks or error handling. Ruby 3.4 introduces Array#fetch_values to retrieve multiple elements safely with default value support.
Before Ruby 3.4
When accessing multiple array indices, you had to choose between unsafe access with values_at (which returns nil for out-of-bounds indices) or multiple fetch calls:
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# Using values_at - returns nil for missing indices
config = ['production', 'us-east-1', '5432']
env, region, port, backup_port = config.values_at(0, 1, 2, 5)
# => ['production', 'us-east-1', '5432', nil]
# Silent failure - backup_port is nil
# Using multiple fetch calls - verbose and repetitive
env = config.fetch(0)
region = config.fetch(1)
port = config.fetch(2)
backup_port = config.fetch(5, '5433') # Default value
This pattern became even more cumbersome when you needed different default values for different indices or wanted consistent error handling across multiple accesses.
Ruby 3.4
Array#fetch_values provides a single method to fetch multiple elements with the same safety guarantees as fetch:
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config = ['production', 'us-east-1', '5432']
# Fetch multiple values safely
config.fetch_values(0, 1, 2)
# => ['production', 'us-east-1', '5432']
# Raises IndexError for out-of-bounds indices
config.fetch_values(0, 1, 5)
# IndexError (index 5 outside of array bounds: -3...3)
# Use a default value for all missing indices
config.fetch_values(0, 1, 2, 5) { |index| 'default' }
# => ['production', 'us-east-1', '5432', 'default']
# Negative indices work as expected
config.fetch_values(-3, -2, -1)
# => ['production', 'us-east-1', '5432']
The block form allows dynamic default values based on the index:
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headers = ['Name', 'Email', 'Role']
# Generate placeholder text for missing columns
headers.fetch_values(0, 1, 3, 4) { |index| "Column #{index}" }
# => ['Name', 'Email', 'Column 3', 'Column 4']
# Use the index to compute defaults
data = [100, 200, 300]
data.fetch_values(0, 2, 5, 7) { |index| index * 10 }
# => [100, 300, 50, 70]
Real-World Usage
Parsing CSV headers where certain columns are optional:
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csv_row = ['John Doe', 'john@example.com', 'Engineer']
REQUIRED_INDICES = [0, 1] # Name and email required
OPTIONAL_INDICES = [2, 3, 4] # Role, department, location optional
# Before: Mix of fetch and values_at
required = REQUIRED_INDICES.map { |i| csv_row.fetch(i) }
optional = csv_row.values_at(*OPTIONAL_INDICES)
# Ruby 3.4: Consistent approach
required = csv_row.fetch_values(*REQUIRED_INDICES)
optional = csv_row.fetch_values(*OPTIONAL_INDICES) { 'N/A' }
# => ['John Doe', 'john@example.com'] and ['Engineer', 'N/A', 'N/A']
Extracting configuration values with validation:
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settings = ['api.example.com', '443', 'true']
# Ensure all required settings exist
host, port, ssl_enabled = settings.fetch_values(0, 1, 2)
# Raises IndexError if any setting is missing
# Optional settings with defaults
cache_size, timeout = settings.fetch_values(3, 4) { '1000' }
# => ['1000', '1000']
Key Differences from Array#values_at
- Error handling:
fetch_valuesraisesIndexErrorfor out-of-bounds indices (without a block), whilevalues_atreturnsnil - Default values:
fetch_valuesaccepts a block to provide defaults;values_athas no default mechanism - Consistency: Aligns with
Hash#fetch_valuesbehavior, providing a familiar API
When to Use Array#fetch_values
- Extracting multiple required values that must exist
- Accessing indices where
nilis a valid value (and shouldn’t indicate missing data) - Providing consistent default values for missing elements
- Validating array structure by ensuring required indices are present
Conclusion
Array#fetch_values brings consistency between Hash and Array APIs while solving the common pattern of safely accessing multiple indices. It provides the strictness of fetch with the multi-value convenience of values_at.
