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Common Patterns

Building off of the style guide, here is a list of common patterns that a Leo developer may encounter as well as the recommended code solution.

Conditional Branches

Conditional if else statements in Leo are expensive. It is preferred to use ternary ? : expressions.

if (condition) {
return a
} else {
return b
return condition ? a : b


Ternary expressions are the cheapest form of conditional. We can resolve the first expression and second expression values before evaluating the condition. This is very easy to convert into a circuit because we know that each expression does not depend on information in later statements.

In the original Example, We cannot resolve the return statements before evaluating the condition. As a solution, Leo creates branches in the circuit so both paths can be evaluated.

branch 1, condition = true
return a
branch 2, condition = false
return b

When the input value condition is fetched at proving time, we select a branch of the circuit to evaluate. Observe that the statement return a is repeated in both branches. The cost of every computation within the conditional will be doubled. This greatly increases the constraint numbers and slows down the circuit.