Proc vs lambda differences (arity, return) in Ruby - Visual Side-by-Side Comparison

Concept Flow - Proc vs lambda differences (arity, return)

Define Proc or Lambda

↓

Call with arguments

↓

Check arity enforcement

↓

Raise error

↓

Execute block body

↓

Return behavior

↓

End

This flow shows how Proc and lambda differ in checking argument count and how return behaves inside them.

Execution Sample

Ruby

def demo
  my_lambda = ->(x) { return x * 2 }
  my_proc = Proc.new { |x| return x * 2 }

  puts my_lambda.call(3)
  puts my_proc.call(3)
end

demo

This code defines a lambda and a proc, each returning double the input, then calls and prints their results.

Execution Table

Step	Code Line	Action	Arity Check	Return Behavior	Output/Effect
1	my_lambda = ->(x) { return x * 2 }	Define lambda expecting 1 argument	Strict: expects exactly 1	Return exits lambda only	Lambda stored
2	my_proc = Proc.new { |x| return x * 2 }	Define proc accepting 1 argument	Lenient: accepts any number	Return exits enclosing method	Proc stored
3	puts my_lambda.call(3)	Call lambda with 3	3 matches 1 arg: OK	Return exits lambda, returns 6	Prints 6
4	puts my_proc.call(3)	Call proc with 3	No strict check, OK	Return exits method, ends method here	Prints 6, then method returns
5	After proc call	Code after proc call not reached	N/A	N/A	No output, program ends

💡 Proc's return exits the whole method, so code after proc call is not executed.

Variable Tracker

Variable	Start	After 1	After 2	Final
my_lambda	undefined	lambda object	lambda object	lambda object
my_proc	undefined	undefined	proc object	proc object
x (lambda)	undefined	undefined	3	3
x (proc)	undefined	undefined	3	3

Key Moments - 3 Insights

Why does the proc cause the method to exit early but the lambda does not?

Because in the execution_table row 4, the proc's return exits the entire method, while the lambda's return only exits the lambda block (row 3).

What happens if you call the lambda with wrong number of arguments?

The lambda enforces arity strictly (row 3), so calling with wrong arguments raises an error, unlike proc which is lenient (row 4).

Does the proc check how many arguments it receives?

No, as shown in row 4, proc accepts any number of arguments without error.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output when calling my_lambda.call(3)?

A 6

B Error due to wrong arity

C 3

D No output

💡 Hint

See execution_table row 3 Output/Effect column

At which step does the program stop executing due to proc's return?

A Step 3

B Step 4

C Step 5

D After Step 5

💡 Hint

Check execution_table row 4 and 5 for return behavior and program end

If you call my_lambda.call(2,3), what happens?

A Returns 4

B Ignores extra argument and returns 4

C Raises an argument error

D Returns nil

💡 Hint

Lambda enforces arity strictly as shown in execution_table row 3

Concept Snapshot

Proc vs Lambda in Ruby:
- Lambda checks exact number of arguments (arity), Proc does not.
- Lambda's return exits only the lambda block.
- Proc's return exits the entire method enclosing it.
- Use lambda when strict argument checking and local return needed.
- Use proc for flexible arguments and non-local return.

Full Transcript

This visual execution trace compares Ruby's Proc and lambda focusing on two key differences: arity checking and return behavior. The flow starts with defining a lambda and a proc, then calling each with one argument. The lambda enforces strict arity, so it expects exactly one argument and returns from the lambda block only. The proc is lenient with arguments and its return exits the whole method, ending execution early. The execution table shows each step, including argument checks and outputs. Variable tracking shows how arguments are passed. Key moments clarify why proc's return exits the method and lambda's does not, and how arity is enforced differently. The quiz tests understanding of output, program stopping point, and arity errors. The snapshot summarizes these differences for quick recall.