"The Department notified us that many U.S. Secret Service (USSS) text messages from January 5 and 6, 2021, were erased as part of a device-replacement program," Inspector General Joseph Cuffari wrote to top members of congressional Homeland Security committees. "The USSS erased those messages after OIG requested records of electronic communications from the USSS, as part of our evaluation of the events at the Capitol on January 6."
Let's describe the motivation behind erased terms with an example. In the following we show a simple state machine which can be in a state On or Off. The machine can change state from Off to On with turnedOn only if it is currently Off. This last constraint is captured with the IsOff[S] contextual evidence which only exists for IsOff[Off]. For example, not allowing calling turnedOn on in an On state as we would require an evidence of type IsOff[On] that will not be found.
erased
Note that in the code above the actual context arguments for IsOff are never used at runtime; they serve only to establish the right constraints at compile time. As these terms are never used at runtime there is not real need to have them around, but they still need to be present in some form in the generated code to be able to do separate compilation and retain binary compatibility. We introduce erased terms to overcome this limitation: we are able to enforce the right constrains on terms at compile time. These terms have no run time semantics and they are completely erased.
As the given evidences of turnedOn and turnedOff are not used in the bodies of those functions we can mark them as erased. This will remove the evidence parameters at runtime, but we would still evaluate the isOn and isOff givens that were found as arguments. As isOn and isOff are not used except as erased arguments, we can mark them as erased, hence removing the evaluation of the isOn and isOff evidences.
erased can also be used as a modifier for a class. An erased class is intended to be used only in erased definitions. If the type of a val definition or parameter is a (possibly aliased, refined, or instantiated) erased class, the definition is assumed to be erased itself. Likewise, a method with an erased class return type is assumed to be erased itself. Since given instances expand to vals and defs, they are also assumed to be erased if the type they produce is an erased class. Finally function types with erased classes as arguments turn into erased function types.
The erased charcoal sign describes the typical appearance of focal prostate cancer in the transition zone characterized as homogeneous hypointensity on T2WI with ill-defined borders, akin to a charcoal pencil drawing smudged with an eraser, often with a lenticular or waterdrop-like shape.
But Rauschenberg was not interested in painting like de Kooning; his plan, as he told the elder artist, was to erase one of his drawings. And de Kooning agreed, after having chosen one that might prove difficult to undo, as it had been done in charcoal, oil paint, pencil, and crayon. Rauschenberg dutifully erased it over the ensuing weeks.
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