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# Dotc's concept of time
Conceptually, the `dotc` compiler's job is to maintain views of
various artifacts associated with source code at all points in time.
But what is *time* for `dotc`? In fact, it is a combination of
compiler runs and compiler phases.
The *hours* of the compiler's clocks are measured in compiler
[runs](https://github.com/lampepfl/dotty/blob/master/src/dotty/tools/dotc/Run.scala). Every
run creates a new hour, which follows all the compiler runs (hours) that
happened before. `dotc` is designed to be used as an incremental
compiler that can support incremental builds, as well as interactions
in an IDE and a REPL. This means that new runs can occur quite
frequently. At the extreme, every keystroke in an editor or REPL can
potentially launch a new compiler run, so potentially an "hour" of
compiler time might take only a fraction of a second in real time.
The *minutes* of the compiler's clocks are measured in phases. At every
compiler run, the compiler cycles through a number of
[phases](https://github.com/lampepfl/dotty/blob/master/src/dotty/tools/dotc/core/Phases.scala).
The list of phases is defined in the [Compiler]object
There are currently about 60 phases per run, so the minutes/hours
analogy works out roughly. After every phase the view the compiler has
of the world changes: trees are transformed, types are gradually simplified
from Scala types to JVM types, definitions are rearranged, and so on.
Many pieces in the information compiler are time-dependent. For
instance, a Scala symbol representing a definition has a type, but
that type will usually change as one goes from the higher-level Scala
view of things to the lower-level JVM view. There are different ways
to deal with this. Many compilers change the type of a symbol
destructively according to the "current phase". Another, more
functional approach might be to have different symbols representing
the same definition at different phases, which each symbol carrying a
different immutable type. `dotc` employs yet another scheme, which is
inspired by functional reactive programming (FRP): Symbols carry not a
single type, but a function from compiler phase to type. So the type
of a symbol is a time-indexed function, where time ranges over
compiler phases.
Typically, the definition of a symbol or other quantity remains stable
for a number of phases. This leads us to the concept of a
[period](https://github.com/lampepfl/dotty/blob/master/src/dotty/tools/dotc/core/Periods.scala).
Conceptually, period is an interval of some given phases in a given
compiler run. Periods are conceptually represented by three pieces of
information
- the ID of the current run,
- the ID of the phase starting the period
- the number of phases in the period
All three pieces of information are encoded in a value class over a 32 bit integer.
Here's the API for class `Period`:
```scala
class Period(val code: Int) extends AnyVal {
def runId: RunId // The run identifier of this period.
def firstPhaseId: PhaseId // The first phase of this period
def lastPhaseId: PhaseId // The last phase of this period
def phaseId: PhaseId // The phase identifier of this single-phase period.
def containsPhaseId(id: PhaseId): Boolean
def contains(that: Period): Boolean
def overlaps(that: Period): Boolean
def & (that: Period): Period
def | (that: Period): Period
}
```
We can access the parts of a period using `runId`, `firstPhaseId`,
`lastPhaseId`, or using `phaseId` for periods consisting only of a
single phase. They return `RunId` or `PhaseId` values, which are
aliases of `Int`. `containsPhaseId`, `contains` and `overlaps` test
whether a period contains a phase or a period as a sub-interval, or
whether the interval overlaps with another period. Finally, `&` and
`|` produce the intersection and the union of two period intervals
(the union operation `|` takes as `runId` the `runId` of its left
operand, as periods spanning different `runId`s cannot be constructed.
Periods are constructed using two `apply` methods:
```scala
object Period {
/** The single-phase period consisting of given run id and phase id */
def apply(rid: RunId, pid: PhaseId): Period }
/** The period consisting of given run id, and lo/hi phase ids */
def apply(rid: RunId, loPid: PhaseId, hiPid: PhaseId): Period
}
```
As a sentinel value there's `Nowhere`, a period that is empty.
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