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# The kairos language
Kairos features a parser frontend which can be used to parse user input into
either a `TimeType` or an `UserIterator`.
This is the documentation for the language this parser understands.
## Basics
There is only addition and subtraction
```
operator = "+" | "-"
```
Numbers are valid, too.
```
digit = "0" | "1" "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"
number = digit+
```
## Units
A unit specifies the Unit of a number
```
second_unit = "seconds" | "second" | "secs" | "sec" | "s"
minute_unit = "minutes" | "minute" | "mins" | "min"
hour_unit = "hours" | "hour" | "hrs" | "hr"
day_unit = "days" | "day" | "d"
week_unit = "weeks" | "week" | "w"
month_unit = "months" | "month"
year_unit = "years" | "year" | "yrs"
unit =
second_unit |
minute_unit |
hour_unit |
day_unit |
week_unit |
month_unit |
year_unit |
```
## Aliases
An alias can be used for specifying "one of a <unit>". For example "1second"
can be specified as "secondly"
```
second_alias = "secondly"
minute_alias = "minutely"
hour_alias = "hourly"
day_alias = "daily"
week_alias = "weekly"
month_alias = "monthly"
year_alias = "yearly"
```
## Amounts
An amount is a number plus a unit, or an alias
```
second_amount = number second_unit | second_alias
minute_amount = number minute_unit | minute_alias
hour_amount = number hour_unit | hour_alias
day_amount = number day_unit | day_alias
week_amount = number week_unit | week_alias
month_amount = number month_unit | month_alias
year_amount = number year_unit | year_alias
amount =
second_amount |
minute_amount |
hour_amount |
day_amount |
week_amount |
month_amount |
year_amount
```
## Exact dates
Exact dates can be specified in ISO 8601 format. The time is optional.
Aliases for today, yesterday and tomorrow exist.
```
two_digits = digit digit
offset = "+" two_digits two_digits
time = two_digits (":" two_digits (":" two_digits offset?)?)?
exact_date =
two_digits two_digits ("-" two_digits ("-" two_digits ("T" time)?)?)? |
"today" |
"yesterday" |
"tomorrow" |
```
As you see, specifying only a year, a year and a month, a date, a date with an
hour, an date with an hour and a minute or a complete year-to-second is
possible as well as specifying an offset ("+0200" for example).
## Expressions
Expressions are calculations of time. There are different _kinds_ of
expressions: Simple adding and subtracting of time amounts,
```
amount_expression = amount (operator amount_expression)?
exact_date_expression = exact_date (operator amount_expression)?
```
## TimeType
A TimeType is either a date or an amount expression:
```
timetype = exact_date_expression | amount_expression
```
That means that a TimeType either represents a specific DateTime or an amount
of time (like "Two weeks" for example).
## Specifying iterators
An iterator consists of three parts which are explained in the following.
### Until
An iterator may have an "until" specification. This tells the iterator when to
stop the iteration.
```
until_spec = "until" exact_date | number "times"
```
**Warning:** The "exact_date" in this variant must be specified from year to
day. Specifying only the year does _not_ work as expected.
This is a known bug.
### Iter specification
The "iter specification" tells the iterator in what steps to iterate.
Aliases for one of a certain unit exists:
```
iter_spec =
"secondly" |
"minutely" |
"hourly" |
"daily" |
"weekly" |
"monthly" |
"yearly" |
number unit
```
### Iterator
The iterator itself is rather simple now:
```
iterator = date iter_spec until_spec?
```
If the `until_spec` is not given, the iterator yields new TimeType objects
**until it panics**.
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