Continuity
Fenl expressions are either discrete or continuous. Discrete expressions describe values that are only meaningful at specific times. Continuous expressions describe values that are meaningful over ranges of time.
Discrete Expressions
A discrete expression produces values at a finite set of times, at all
other times its value is null
. Tables contain a finite set of events,
and each event is associated with a specific time. A table only has a a
meaningful value at the set of times for which events exist. Similarly,
simple operations such as field reference produce discrete results when
applied to discrete expressions.
Discrete expressions may be combined in various ways, for example by adding them together as shown.
In most cases operations produce output values any time one of their
input expressions produces a value. In this example we can see that many
of the values produced are null
, since the expression
Purchase.amount
is defined at different times than Review.stars
, and
adding null
to a number produces null
.
Continuous Expressions
A continuous expression has a meaningful value at all points in time. Continuous expressions produce values at every time contributing to their value. Aggregations generally produce continuous values.
For example, at any point in time we can describe the sum of all
Purchase.amount
values seen so far.
Continuous expressions have a value at all times, but they produce values at specific times. These times are illustrated above as circles. A continuous expression can be thought of as producing a stream of updates; at any given time the stream takes the value of the most recent update.
When continuous values are aggregated, the aggregation is updated when a
value is produced. For example, count
produces the same result for a
discrete expression with and without an intermediate aggregation.
Purchase.amount  count() == Purchase.amount  sum()  count()
Design
The distinction between "having" a value and "producing"
a value allows aggregations to be applied in the same way to both
continuous and discrete expression. Applying an operation such as We feel interpreting aggregations of continuous values as integration
would be surprising, especially in cases such as 
Interactions Between Continuous and Discrete Expressions
The continuity of the result of an operation involving multiple
expressions depends on the continuity of the operation’s inputs. In
general discreteness is "greedy". Given discrete expressions defined at
the set of times A
and B
and a continuous expression, the result of
a binary operation between any pair is described in the following table.
Discrete[A] 
Discrete[B] 
Continuous 














When a discrete expression is combined with a continuous expression, each the value at time present in the discrete expression is combined with the value the continuous expression has at that same time to produce a result.
Filtering when Values are Produced
As mentioned, most operations between expressions produce a value when
any input expression produces a value. This can lead to values being
produced at undesirable times. The when
operation allows filtering the
times at which an expression produces values. It works by producing the
value of an expression whenever a predicate expression produces the
value true
.