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4.3 TFM Compliance Problems

TFM is very thorough in the checks it performs while reading TFM data, to the point that it could probably be used as a validation tool. Every compliance problem detected will be signalled with a warning or an error, depending on the seriousness of the problem (in fact, only two compliance conditions are warnings currently; all others being errors). At the same time, TFM also tries to offer a way out of problematic situations (in the form of restarts, currently non-interactive), when it sounds reasonable. This means that it is almost always possible to load a font, however broken it may be.

When loading a file, a restart named cancel-loading is always active. If invoked, load-font will just abort and return nil. Otherwise, the available restarts depend on the dynamic context and the particular condition being signalled. The graph on the next page depicts the conditions / restarts ontology related to compliance problems. Green boxes denote warnings, red ones denote errors, and blue ellipses denote restarts. By following a branch from left to right in this graph, you get an idea of which restarts are available when a specific condition is signalled. They are ordered by decreasing proximity to the signalling code (or, to put it differently, by increasing level of abstraction). The conditions in this graph are described precisely in Exported conditions in The TFM Reference Manual. The restarts, on the other hand, are not, but their names should be self-explanatory. Note that a branch in this graph does not necessarily correspond to a specific code path in the library. Different code paths may have different restarts with the same name when it makes sense, but eponymous restarts are “merged” into a single node in the graph.

One particular case deserves some additional bits of information though. You will notice the existence of both an invalid-design-size error, and an invalid-original-design-size warning. Remember that these do not concern user-provided design sizes, which may only trigger a type error (see Erroneous Usage). In the default case, the invalid-design-size error is signalled when a TFM file is not compliant. If, however, you have call load-font with an alternative :design-size, you are effectively overriding the original value, which will not be used. In that case, only a warning is signalled: the invalid-original-design-size warning. Finally, please consider that leaving that warning alone means that an invalid original design size will be kept around in the font, so if you plan on using it later on, you’re looking for trouble.


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