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Conditional expressions

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Conditional expressions consist of three sub-expressions, where the left-most sub-expression is of type Bool and determines which one of the two other sub-expressions is evaluated. They are of the form

cond ? ifTrue | ifFalse

Specifically, if cond evaluates to true, then the conditional expression evaluates to the ifTrue expression; otherwise, it evaluates to the ifFalse expression. The other expression (the ifFalse and ifTrue expression, respectively) is never evaluated, much like the branches in an if statement. For instance, in an expression a == b ? C(qs) | D(qs), if a equals b, then the callable C is invoked. Otherwise, D is invoked.

The types of the ifTrue and the ifFalse expression have to have a common base type. Independent of which one ultimately yields the value to which the expression evaluates, its type always matches the determined base type.

For example, if

  • Op1 is of type Qubit[] => Unit is Adj
  • Op2 is of type Qubit[] => Unit is Ctl
  • Op3 is of type Qubit[] => Unit is Adj + Ctl

then

  • cond ? Op1 | Op2 is of type Qubit[] => Unit
  • cond ? Op1 | Op3 is of type Qubit[] => Unit is Adj
  • cond ? Op2 | Op3 is of type Qubit[] => Unit is Ctl

For more details, see subtyping.