How to use [if]

The reference page of if has its arguments, constraints and declaration.

This is the Hyperlambda equivalent of if from other programming languages. It allows you to test for some condition, and evaluate a lambda object, only if the condition evaluates to true. [if] must be given two arguments. The first argument can be anything, including a slot invocation - But its second argument must be its [.lambda] argument. The [.lambda] node will be evaluated as a lambda object, only if the first argument to [if] evaluates to boolean true. Below is an example.

.dest
if
   .:bool:true
   .lambda
      set-value:x:@.dest
         .:yup!

All conditional slots, including [if], optionally accepts slots as their first condition argument. This allows you to invoke slots, treating the return value of the slot as the condition deciding whether or not the [.lambda] object should be executed or not. Below is an example.

.arguments
   foo:bool:true
.dest

if
   get-value:x:@.arguments/*/foo
   .lambda
      set-value:x:@.dest
         .:yup!

Notice, both the [if] slot and the [else-if] slot can optionally be directly pointing to an expression, that is assumed to evaluate to either boolean true or boolean false, such as the following illustrates.

.arguments
   foo:bool:true
.dest

if:x:@.arguments/*/foo
   set-value:x:@.dest
      .:yup!

If you use this shorthand version for the slot(s), its lambda object is assumed to be the entirety of the content of the [if] or [else-if] slot itself, and there are no needs to explicitly declare your lambda objects as a [.lambda] argument. This only works if the expression leads to a boolean value.