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When we would expect the compiler to have the instance already available, in actual fact we have to provide it ourselves. The problem is that the constraint is not actually on the datatype, but rather, on the type of the constructor. Worse, the dictionary is not stored in the constructor, but simply discarded.
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### Effect of removing datatype contexts
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It is possible to write programs whose behaviour changes when datatype contexts are removed, e.g. with:
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