Lambda expressions

Overview

DFS uses lambda expressions to define transformations on data points as well as define Boolean conditions that act as filters. Lambda expressions wrap mathematical operations, Boolean operations, internal function calls or a combination of all three.

All lambda expressions in DFS begin with the lambda: keyword.

|where(lambda: "topic" == 'ttop/grap/prec')

In the above example "topic" is used to access the value of a field called topic from the current data_point and compared against the string 'ttop/grap/prec'.

Note here that literal string values are declared using single quotes, while double quotes are used to access the values of tags and fields within the current data_item.

Field paths

As field and tag values can be deeply nested maps and lists, it is possible to use a JSON-path like syntax to reference them:

Valid examples:

"averages"

"axis.z.cur"

"value.sub[2].data"

"averages.emitted[5]"

Built-in functions

Type conversion

With a few exceptions every type can be converted to every other type.

Bool

bool(a_value) -> true|false

Integer

int(value) -> integer

Float

float(value) -> float

String

string(val) -> string

Time functions

Every data_point in faxe contains a field called ts, which holds a UTC timestamp in milliseconds.

Function	Description
`now()` -> integer	returns an utc timestamp in milliseconds
`to_iso8601(ts)` -> string	converts the timestamp to an ISO8601 datetime string
`to_rfc3339(ts)` -> string	converts the timestamp to an RFC3339 datetime string
`millisecond(ts)` -> integer	milliseconds within the second [0, 999]
`second(ts)` -> integer	second within the minute [0, 59]
`minute(ts)` -> integer	minute within the hour [0, 59]
`hour(ts)` -> integer	hour within the day [0, 23]
`day(ts)` -> integer	day within the month [1, 31]
`day_of_week(ts)` -> integer	the weekday with week [1, 7] 1 is monday
`week(ts)` -> integer	isoweek-number within year [1, 53]
`month(ts)` -> integer	month within the year [1, 12]

Example:

lambda: hour("ts") >= 8 AND hour("ts") < 19

The above expression evaluates to true if the hour of the day for the data point falls between 08:00 and 19:00.

Math functions

Function	Description
`abs(x)` -> number
`acos(x)` -> float
`acosh(x)` -> float
`asin(x)` -> float
`asinh(x)` -> float
`atan(x)` -> float
`atan2(y, x)` -> float
`atanh(x)` -> float
`ceil(x)` -> float
`cos(x)` -> float
`cosh(x)` -> float
`exp(x)` -> float
`floor(x)` -> float
`fmod(x, y)` -> float
`log(x)` -> float
`log10(x)` -> float
`log2(x)` -> float
`max(x, y)` -> number
`max(list)` -> number
`min(x, y)` -> number
`min(list)` -> number
`pi() -> float`	gives pi
`pow(x, y)` -> float
`round(x)` -> integer	round a number to an integer
`round_float(x, precision)` -> float	round a float (x) with the given precision
`sin(x)` -> float
`sinh(x)` -> float
`sqrt(x)` -> float
`tan(x)` -> float
`tanh(x)` -> float

String functions

String positions start with index 0.

Function	Description
`str_at(x, pos)` -> string/undefined	Returns the grapheme in the position of the given utf8 string. If position is greater than string length, then it returns undefined. Negative offsets count back from the end of the string.
`str_capitalize(x)` -> string	Converts the first character in the given string to uppercase and the remaining to lowercase
`str_contains(x, contents)` -> bool	Check if string contains any of the given contents
`str_downcase(x)` -> string	Convert all characters on the given string to lowercase
`str_ends_with(x, suffix)` -> string	Returns true if string ends with suffix, otherwise false.
`str_ends_with_any(x, suffixes)` -> string	Returns true if string ends with any of the suffixes given, otherwise false.
`str_eqi(x,y)` -> bool	Compares strings case insensitively
`str_first(x)` -> string/undefined	Returns the first grapheme from an utf8 string, undefined if the string is empty
`str_last(x)` -> string/undefined	Returns the last grapheme from an utf8 string, undefined if the string is empty
`str_length(x)` -> int	Returns the number of unicode graphemes in an utf8 string
`str_lstrip(x)` -> string	Returns a string where leading Unicode whitespace has been removed
`str_lstrip(x, char)` -> string	Returns a string where leading char have been removed
str_normalize/2
str_pad_leading/2
str_pad_leading/3
str_pad_trailing/2
str_pad_trailing/3
`str_replace(x, patt, repl)` -> string	Returns a new string based on subject by replacing the parts matching pattern by replacement.
str_replace_leading/3	Replaces all leading occurrences of match by replacement of match in string.
str_replace_trailing/3	Replaces all trailing occurrences of match by replacement of match in string.
`str_replace_prefix(x, match, repl)` -> string	Replaces prefix in string by replacement if it matches match. Returns the string untouched if there is no match. If match is an empty string (""), replacement is just prepended to string.
`str_replace_suffix(x, match, repl)` -> string	Replaces suffix in string by replacement if it matches match. Returns the string untouched if there is no match. If match is an empty string (""), replacement is just appended to string.
`str_reverse(x)` -> string	Reverses the given string.
`str_rstrip(x)` -> string	Returns a string where trailing Unicode whitespace has been removed
`str_rstrip(x, char)` -> string	Returns a string where trailing char have been removed
`str_slice(x, start, len)` -> string	Returns a substring starting at the offset given by the first, and a length given by the second param, if offset is negative, count back from end of string.
str_split/1
str_split/2
str_split/3
str_split_at/2
str_split_by_any/2
str_split_by_any/3
str_split_by_re/2
str_split_by_re/3
`str_starts_with(x, pre)` -> bool	Returns true if string starts with Prefix
`str_starts_with_any(x, prefixes)` -> bool	Returns true if string starts with any of the prefixes given, otherwise false.
`str_strip(x)` -> string	Returns a string where leading/trailing Unicode whitespace has been removed
`str_strip(x, char)` -> string	Returns a string where leading/trailing char have been removed
`str_upcase(x)` -> string	Convert all characters on the given string to uppercase

Misc

Function	Description	Example
`defined(Key)` -> bool	whether the given Key is defined in the current data-item
`undefined(Key)` -> bool	whether the given Key is NOT defined in the current data-item
`topic_part(TopicString, PartIndex, [Seperator])` -> string	extract a part from a topic string, Separator defaults to '/', the index of the first part is `1` not 0	topic_part('this/is/mytopic', 2) -> 'is'
`member(Ele, ListOrMap)` -> bool	check for list/set membership of a value, or when used with a map, check if Ele is a key in the map
`not_member(Ele, List)` -> bool
`map_get(Key, Map)` -> any	get a value from a map, 'undefined' is returned, if the key is not present in map	map_get("topic", ls_mem('stream_lookup'))
`random(N)` -> integer	generate a random integer between 1 and N
`random_real(N)` -> float	generate a random float between 0.0 and 1.0, that gets multiplied by N
`mem` values are set with the mem node
`ls_mem(Key)` -> any	get the single value associated with Key from the flow-memory
`ls_mem_list(Key)` -> any	get the list value associated with Key from the flow-memory
`ls_mem_set()` -> any	get the set value associated with Key from the flow-memory

Conditional functions

If

Returns the result of its operands depending on the value of the first argument. The second and third arguments must return the same type.

Example:

|eval(lambda: if("field.val1" > threshold AND "field.val1" != 0, 'true', 'false'))
.as('value')

The value of the field value in the above example will be the string true or false, depending on the condition passed as the first argument.

The if function’s return type is the same type as its second and third arguments.

if(condition, true expression, false expression)