The unit value.

An integer.

Literal text.

A direction.

A boolean.

A reference to a robot.

An injection into a sum type. False = left, True = right.

A pair.

A closure, representing a lambda term along with an environment containing bindings for any free variables in the body of the lambda.

An application of a constant to some value arguments, potentially waiting for more arguments. If a constant application is fully saturated (as defined by its arity), whether it is a value or not depends on whether or not it represents a command (as defined by isCmd). If a command (e.g. Build), it is a value, and awaits an FExec frame which will cause it to execute. Otherwise (e.g. If), it is not a value, and will immediately reduce.

A definition, which does not take effect until executed. The Bool indicates whether the definition is recursive.

The result of a command, consisting of the result of the command as well as an environment of bindings from TDef commands.

An unevaluated bind expression, waiting to be executed, of the form i.e. c1 ; c2 or x <- c1; c2. We also store an Env in which to interpret the commands.

A (non-recursive) delayed term, along with its environment. If a term would otherwise be evaluated but we don't want it to be (e.g. as in the case of arguments to an 'if', or a recursive binding), we can stick a TDelay on it, which turns it into a value. Delayed terms won't be evaluated until Force is applied to them.

A reference to a memory cell in the store.

A record value.

A keyboard input.

A requirements command awaiting execution.