How to use [map]

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

[map] iterates through each node being the result of its expression, the same way [for-each] does, and evaluates its lambda object once for each node, passing in the currently iterated node as a [.dp] argument. The value or nodes returned by the lambda object for each iteration, becomes the result of the slot. Each returned value becomes a . node, and any returned child nodes are added as they are. After evaluation, the children of [map] are replaced with the mapped result.

• [map] - Expression selecting the nodes to map
• Children - Lambda object evaluated once per selected node, and expected to return a value or nodes, for instance using [return] or [yield]

.src
   .:int:1
   .:int:2
   .:int:3
map:x:@.src/*
   math.increment:x:@.dp/#
   unwrap:x:+/*
   yield
      .:x:@math.increment

After execution, [map] has 3 children, with the values 2, 3 and 4. If an iteration returns neither a value nor any nodes, the slot throws an exception.