fix #34 minimal time manipulation

docs/time.md

@@ -0,0 +1,123 @@
+# Lyng time functions
+
+Lyng date and time support requires importing `lyng.time` packages. Lyng uses simple yet modern time object models:
+
+- `Instant` class for time stamps with platform-dependent resolution
+- `Duration` to represent amount of time not depending on the calendar, e.g. in absolute units (milliseconds, seconds, hours, days)
+
+## Time instant: `Instant`
+
+Represent some moment of time not depending on the calendar (calendar for example may b e changed, daylight saving time can be for example introduced or dropped). It is similar to `TIMESTAMP` in SQL or `Instant` in Kotlin. Some moment of time; not the calendar date.
+
+Instant is comparable to other Instant. Subtracting instants produce `Duration`, period in time that is not dependent on the calendar, e.g. absolute time period.
+
+It is possible to add or subtract `Duration` to and from `Instant`, that gives another `Instant`.
+
+Instants are converted to and from `Real` number of seconds before or after Unix Epoch, 01.01.1970. Constructor with single number parameter constructs from such number of seconds,
+and any instance provide `.epochSeconds` member:
+
+    import lyng.time
+
+    // default constructor returns time now:
+    val t1 = Instant()
+    val t2 = Instant()
+    assert( t2 - t1 < 1.millisecond )
+    assert( t2.epochSeconds - t1.epochSeconds < 0.001 )
+    >>> void
+
+## Constructing
+
+    import lyng.time
+
+    // empty constructor gives current time instant using system clock:
+    val now = Instant()
+
+    // constructor with Instant instance makes a copy:
+    assertEquals( now, Instant(now) )
+
+    // constructing from a number is trated as seconds since unix epoch:
+    val copyOfNow = Instant( now.epochSeconds )
+
+    // note that instant resolution is higher that Real can hold
+    // so reconstructed from real slightly differs:
+    assert( abs( (copyOfNow - now).milliseconds ) < 0.01 )
+    >>> void
+
+The resolution of system clock could be more precise and double precision real number of `Real`, keep it in mind.
+
+## Comparing and calculating periods
+
+    import lyng.time
+    
+    val now = Instant()
+    
+    // you cam add or subtract periods, and compare
+    assert( now - 5.minutes < now )
+    val oneHourAgo = now - 1.hour
+    assertEquals( now, oneHourAgo + 1.hour)
+
+    >>> void
+
+## Instant members
+
+| member                | description                                             |
+|-----------------------|---------------------------------------------------------|
+| epochSeconds: Real    | positive or negative offset in seconds since Unix epoch |
+| isDistantFuture: Bool | true if it `Instant.distantFuture`                      |
+| isDistantPast: Bool   | true if it `Instant.distantPast`                        |
+
+## Class members
+
+| member                         | description                                             |
+|--------------------------------|---------------------------------------------------------|
+| Instant.distantPast: Instant   | most distant instant in past                            |
+| Instant.distantFuture: Instant | most distant instant in future                          |
+
+# `Duraion` class
+
+Represent absolute time distance between two `Instant`.
+
+    import lyng.time
+    val t1 = Instant()
+
+    // yes we can delay to period, and it is not blocking. is suspends!
+    delay(1.millisecond)
+
+    val t2 = Instant()
+    // be suspend, so actual time may vary:
+    assert( t2 - t1 >= 1.millisecond)
+    assert( t2 - t1 < 100.millisecond)
+    >>> void
+
+Duration can be converted from numbers, like `5.minutes` and so on. Extensions are created for
+`Int` and `Real`, so for n as Real or Int it is possible to create durations::
+
+- `n.millisecond`, `n.milliseconds`
+- `n.second`, `n.seconds`
+- `n.minute`, `n.minutes`
+- `n.hour`, `n.hours`
+- `n.day`, `n.days`
+
+The bigger time units like months or years are calendar-dependent and can't be used with `Duration`.
+
+Each duration instance can be converted to number of any of these time units, as `Real` number, if `d` is a `Duration` instance:
+
+- `d.milliseconds`
+- `d.seconds`
+- `d.minutes`
+- `d.hours`
+- `d.days`
+
+for example
+
+    import lyng.time
+    assertEquals( 60, 1.minute.seconds )
+    >>> void
+
+# Utility functions
+
+## delay(duration: Duration)
+
+Suspends current coroutine for at least the specified duration.
+
+

docs/tutorial.md

@@ -14,7 +14,7 @@ __Other documents to read__ maybe after this one:
 - [Advanced topics](advanced_topics.md), [declaring arguments](declaring_arguments.md)
 - [OOP notes](OOP.md), [exception handling](exceptions_handling.md)
 - [math in Lyng](math.md)
-- Some class references: [List], [Set], [Map], [Real], [Range], [Iterable], [Iterator]
+- Some class references: [List], [Set], [Map], [Real], [Range], [Iterable], [Iterator], [time manipulation](time.md)
 - Some samples: [combinatorics](samples/combinatorics.lyng.md), national vars and loops: [сумма ряда](samples/сумма_ряда.lyng.md). More at [samples folder](samples)
 
 # Expressions
@@ -668,7 +668,6 @@ are [Iterable]:
 
 Please see [Map] reference for detailed description on using Maps.
 
-
 # Flow control operators
 
 ## if-then-else
@@ -1101,6 +1100,17 @@ and you can use ranges in for-loops:
 
 See [Ranges](Range.md) for detailed documentation on it.
 
+# Time routines
+
+These should be imported from [lyng.time](time.md). For example:
+
+    import lyng.time
+
+    val now = Instant()
+    val hourAgo = now - 1.hour
+
+See [more docs on time manipulation](time.md)
+
 # Comments
 
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