Conditionals

Conditional statements let you choose to execute a block of code, or only when certain conditions are met. For example, if it’s warmer than 68 degrees outside, then you might want to go for a jog; otherwise, you might want to stay indoors and play cards.

if (temperature > 68) then {
    print "Go jogging!"
} else {
    print "Play cards instead."
}

Note the structure here. In Grace, if statements start with if followed by an expression that evaluates to true or false. This is called a Boolean expression, or sometimes a just a condition. The Boolean expression is followed by then, and a block of code that runs only if the Boolean expression is true.
The else statement is optional, and is followed by a code block that runs only if the expression is false.

Note that Boolean expressions can be also used outside of conditionals. For example, you can declare var b := false.

Feel free to change the value of spoonSize in the example program below, and see the results for yourself.

Conditional Expressions

The spoonSize example illustrates the conditional statement; one of the two print statements is executed, depending on the condition. You can also use an if(_)then(_)else(_) as an expression, that is, so that it returns a value rather than executing a statement. Here is the same program using a conditional expression.

Notice that the else(_) part of the conditional is compulsory in a conditional expression, even though it was optional in the conditional statement. This makes sense, if you think about it. Omitting the else(_) in a conditional statement is equivalent to saying “do nothing”. But a conditional expression has to return a value, so both the then(_) and else(_) must also return values.

elseif(_)

If you want to check more conditions, the simplest way is to use an extended form of if(_)then(_)else(_) that contains one or more elseif(_) clauses. The condition after an elseif must be in braces; it is evaluated only when the preceding conditions are all false. Here is an example.