Captures are the basic frame of execution in Lasso. All code that executes does so within a capture. When a method is invoked, a capture is first automatically created for that method to execute in. When executing code in a source file, a capture is again automatically created for that code to execute in.

Captures are everywhere in Lasso, and learning how to use them will give you a powerful tool to use for solving some complex problems. This chapter provides in-depth information about captures and examples of their use.

Capture Structure

A capture is a representation of the control state of a section of code. While methods’ code blocks are stateless (once they have had their code established), captures maintain state, some of which may change frequently during execution. This state consists of:

  • The current method’s code
  • The current “self” and “inherited”
  • The current “params” staticarray
  • The current set of local variables, and their values
  • The current program counter, or “PC”. This value is the offset within that capture’s code at which execution is currently happening.
  • The name of the current method call
  • The current continuation, which is the element to be executed after the current capture completes
  • The set of handlers that must be executed before the capture completes
  • A home capture, which is the capture in which this capture was created

When a capture is invoked, it will in turn execute its associated code which will execute within the context of that capture’s state. The currently executing capture is known as the current capture and is made available through the currentCapture method. (See the Operators chapter for more information about invocation.)

Creating Captures

As previously mentioned, captures are automatically created when a method is executed. Captures can also be manually created by using curly braces as an expression. When using the association operator (=>) to invoke an object by passing it a capture, the capture is known as the object’s associated block or capture block.

#ary->forEach => {
   // ... a capture of the surrounding code ...

In the code above, forEach is associated with a capture object. This results in forEach being invoked with the capture as its capture block, which it may execute as needed.

Captures can also be assigned to variables like any other object. The following example creates a capture and assigns it to the variable “cap”:

local(cap) = { /* ... the capture's code ... */ }

There are two types of captures supported in Lasso: regular captures, like the examples above, and auto-collect captures. An auto-collect capture concatenates the result of calling the asString method on every value produced inside the capture when the capture is executed, and produces that value. The following example creates an auto-collect capture and assigns it to the variable “cap”:

local(cap) = {^ /* ... the capture's code ... */ ^}

Because all executing code occurs within a capture, every capture that is manually created (as in the two examples above) is done so within the context of another capture. This surrounding capture is known as the new capture’s home capture. Not all captures will have a home. Captures created automatically based on the invocation of a method will not have a home. A capture that is created within a capture that does have a home will have its home set to its parent capture’s home. This means that nested captures will all have the same home.

A capture with a home will always take the following environment values from its home: self, locals, params, and current call name. A capture without a home will have state values based on the circumstances of the call. All other capture state is unique to each capture. As described below, the home capture is important for determining the behavior of return and yield.

Executing Captures

Captures are executed by calling their invoke method:

local(cap) = { /* ... the capture's code ... */ }
#cap->invoke  // Invoke the capture
#cap()        // Shorthand invocation

You can pass parameters to the capture->invoke method, and these are available with the special parameter local variables (#1, #2, etc.):

local(dist) = {
   local(x1) = #1
   local(y1) = #2
   local(x2) = #3
   local(y2) = #4
#dist(8, 2, 10, 5) // Sets #x1, #y1, #x2, #y2 to 8, 2, 10, 5, respectively

When you invoke an auto-collect capture, the auto-collected value will be returned and can be accessed using capture->autoCollectBuffer:

local(distance) = {^
   local(x1) = #1
   local(y1) = #2
   local(x2) = #3
   local(y2) = #4

   math_sqrt(math_pow(math_abs(#x2-#x1), 2) + math_pow(math_abs(#y2-#y1), 2))

#distance(8, 2, 10, 5)
// => 3.605551

// => 3.605551

Stored captures can be executed at any point and the code contained within will operate as if it had been executed in the context in which it was created. This means that it will have access to the surrounding local variables where the capture was created even when the capture is being executed in code that has a different scope. The example below illustrates this by creating a capture in the method1 method whose code is set to update the local variable “my_local” in method1. We then invoke that capture in “method2” which changes the value for “my_local” in method1. Returning “my_local” confirms that the value has been updated by method2.

define method1 => {
   local(my_cap) = {

   #my_local = 'Hello'

   return #my_local

define method2(cap::capture) => {
   #cap(', world.')


// => Hello, world.

Producing Values and Detaching

Captures can produce values by using yield or return. Both yield and return halt the execution of any of the capture’s remaining code and produce the specified value. Yielding from a capture differs from returning in how it leaves the capture. A return will reset the capture’s PC to the top while a yield will not modify the PC. This affects how the capture behaves if it is executed a second time. A capture that has been returned from will begin executing from the start of the capture. A capture that has been yielded from will begin executing immediately after the expression that caused it to yield in the first place. A capture may yield many times.

local(cap) = {
   yield 1
   yield 2
   yield 3
   yield 4

// => 1
// => 2
// => 3
// => 4
// => 1   // Capture reached the end and reset

Note that once a capture reaches its end, the PC will automatically be reset back to the top. (Read on for a discussion of why we use capture->detach here.)

Even though a capture has yielded, it can still elect to return later in the code, thus resetting itself:

#cap = {
   yield  1
   yield  2
   return 3 // Subsequent calls will start from beginning
   yield  4 // This is unreachable

The current home capture is very important for determining the behavior of return and yield. Because captures are intended to execute as if they had been invoked directly within their home, return and yield will both behave by exiting from the current home as well as itself. This is known as a non-local return, and is illustrated in the following example which implements a potential contains method:

define contains(a::array, val) => {
   #a->forEach => {
      #val == #1 ?
         return true // This return is non-local
   return false

Even though the return true occurs within a nested capture that is potentially several levels deep, it causes all intervening captures to halt their execution (with all their handlers executing in the process) up to and including the capture’s home.

A capture can be detached from its home in order to escape from this behavior. The easiest way to accomplish this is to call the capture’s capture->detach method. This method detaches the capture from its home and returns itself as the method’s result. (This is what we did in the first yield example above.)

The following example creates a capture and detaches it from its home. Returning from within the capture no longer exits the surrounding capture.

local(cap) = {
   return self->type

// => // Produces result of self->type

Note that because the capture above is detached, it returns as normal and simply produces its value to the caller and allows the caller to continue its execution. It is not a non-local return.

Captures provide two other forms of yield and return: yieldHome and returnHome. These are only valid when the capture has a home and can be used to return from a capture to its home, instead of returning from its home. These special-purpose forms are used to accomplish some implementation details such as certain looping constructs or other control flow structures. (For example, loop_continue and loop_abort both rely on using these forms.)

Capture Methods


Returns a reference to the capture that is currently executing.

type capture

A capture is a block of Lasso code that can be passed to another method or invoked locally. Captures are context-aware and retain state during execution.


Executes the capture object and the code that is associated with it.


Detaches the capture so that it no longer has a home capture and then returns itself. After this, calling capture->home will return “void”.


Resets the program counter (PC) for the capture and begins executing the capture’s code again.


Returns the capture that will be executed after this capture completes.


Returns the home capture of the current capture object.


Returns the file name where the capture object was defined.


Returns the current line of code that is being executed in the capture object based on the file where the capture was defined.


Returns the current column of code that is being executed in the capture object based on the file where the capture was defined.


Returns the current call stack of the code that is being executed based on where the capture was called. Each line of the call stack consists of a line number, column number, and file name for the capture invocations leading up to the current one. The top of the stack has the most recent capture call and the list works its way back through each call.


Returns the capture block associated with the current capture object, if any.


If the capture is an auto-collect capture, this will store the current auto-collect value created by invoking the capture.


If the capture is an auto-collect capture, this will set the auto-collect value.


If the capture was created to run a method, this will return that method’s name.


Invokes the capture. If it is an auto-collect capture, this will return the auto-collect value, but will not update capture->autoCollectBuffer.