SEXPs

A SEXP (Symbolic EXPression) is the scripting language built into FreeSpace missions. Every condition, trigger, and action in a mission — from checking whether a ship has been destroyed to playing a message — is expressed as a SEXP tree.

Structure

SEXPs use a Lisp-style prefix notation enclosed in parentheses:

(operator argument1 argument2 ...)

Operators can be nested, so arguments can themselves be SEXPs:

(when
    (is-destroyed-delay 0 "Epsilon 1")
    (send-message "" "Mission-Complete" "Command")
)

Here when is the top-level operator. Its first argument is a condition (is-destroyed-delay) and its second argument is an action (send-message). When the condition becomes true the action fires.

Where SEXPs appear

SEXPs are used in:

• Mission Events — the primary place to author SEXP logic; see the Mission Events dialog.
• Mission Goals — the completion condition for each goal; see the Mission Goals dialog.
• Ship arrival/departure cues — conditions on when ships enter and leave the mission.
• Briefing and debriefing — stage-change conditions.

Editing SEXPs in QtFRED

The SEXP tree editor is embedded directly in the Mission Events and Mission Goals dialogs. Right-click any node in the tree to add, replace, or remove operators and arguments. Hover over an operator name for a brief description of what it does.