Removal of useless forms


This utility removes subforms which do nothing. Useless subforms may result from poorly written user-level code. They are also created if user-level code discards the value produced by a form such as scan, which generates both side-effects and a value. Finally, certain rewriting rules within the compiler must preserve subforms that may or may not be useful, because they do not know if they will generate side-effects or whether a form is or is not pure.

Removal of useless forms must follow type inference, which wraps a return form around the code if it returns a value to the caller.

Specifically, a subform is considered useful if it either side-effects the world (including setting final return values if this function returns any). Recursively, subforms are useful if they produce values used by useful subforms. Forms that side-effect the world include

All other subforms are useless. Furthermore, forms following an unconditional call to (icp-error) are useless, because they can never execute.

All useless forms are removed from the code. An error is triggered if the entire function is found to do nothing.


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