ICR transformation does “source to source” transformations on known global functions, taking advantage of semantic information such as argument types and constant arguments. Transformation is optional, but should be done if speed or space is more important than compilation speed. Transformations which increase space should pass when space is more important than speed.
A transform is actually an inline function call where the function is computed at compile time. The transform gets to peek at the continuations for the arguments, and computes a function using the information gained. Transforms should be cautious about directly using the values of constant continuations, since the compiler must preserve eqlness of named constants, and it will have a hard time if transforms go around randomly copying constants.
The lambda that the transform computes replaces the original function variable reference as the function for the call. This lets the compiler worry about evaluating each argument once in the right order. We want to be careful to preserve type information when we do a transform, since it may be less than obvious what the transformed code does.
There can be any number of transforms for a function. Each transform is associated with a function type that the call must be compatible with. A transform is only invoked if the call has the right type. This provides a way to deal with the common case of a transform that only applies when the arguments are of certain types and some arguments are not specified. We always use the derived type when determining whether a transform is applicable. Type check is responsible for setting the derived type to the intersection of the asserted and derived types.
If the code in the expansion has insufficient explicit or implicit argument type checking, then it should cause checks to be generated by making declarations.
A transformation may decide to pass if it doesn’t like what it sees when it looks at the args. The Give-Up function unwinds out of the transform and deals with complaining about inefficiency if speed is more important than brevity. The format args for the message are arguments to Give-Up. If a transform can’t be done, we just record the message where ICR finalize can find it. note. We can’t complain immediately, since it might get transformed later on.