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Users can now register string notations for custom inductives.
Much of the code and documentation was copied from numeral notations.
I chose to use a 256-constructor inductive for primitive string syntax
because (a) it is easy to convert between character codes and
constructors, and (b) it is more efficient than the existing `ascii`
type.
Some choices about proofs of the new `byte` type were made based on
efficiency. For example, https://github.com/coq/coq/issues/8517 means
that we cannot simply use `Scheme Equality` for this type, and I have
taken some care to ensure that the proofs of decidable equality and
conversion are fast. (Unfortunately, the `Init/Byte.v` file is the
slowest one in the prelude (it takes a couple of seconds to build), and
I'm not sure where the slowness is.)
In String.v, some uses of `0` as a `nat` were replaced by `O`, because
the file initially refused to check interactively otherwise (it
complained that `0` could not be interpreted in `string_scope` before
loading `Coq.Strings.String`).
There is unfortunately a decent amount of code duplication between
numeral notations and string notations.
I have not put too much thought into chosing names; most names have been
chosen to be similar to numeral notations, though I chose the name
`byte` from
https://github.com/coq/coq/issues/8483#issuecomment-421671785.
Unfortunately, this feature does not support declaring string syntax for
`list ascii`, unless that type is wrapped in a record or other inductive
type. This is not a fundamental limitation; it should be relatively
easy for someone who knows the API of the reduction machinery in Coq to
extend both this and numeral notations to support any type whose hnf
starts with an inductive type. (The reason for needing an inductive
type to bottom out at is that this is how the plugin determines what
constructors are the entry points for printing the given notation.
However, see also https://github.com/coq/coq/issues/8964 for
complications that are more likely to arise if inductive type families
are supported.)
N.B. I generated the long lists of constructors for the `byte` type with
short python scripts.
Closes #8853
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