MapCSS/0.2/eval

This page contains proposed MapCSS eval() syntax.

Data types

EIAS: Everything is a string.

When used in different contexts, it may be considered as:

• string

String is enclosed in "quotation marks". It can be:

• concatenated using . operator.
• checked for being a number using num() function. In case it can't be converted, "" (empty string) is returned.
• converted to a number using metric() or zmetric() function. These functions should return (heuristically?) parsed width/height in map units. E.g. on "2 meters in 1 pixel" scale metric("3m") should return 1.5. zmetric() does the same for elevations. In the case it can't be converted, "" (empty string) should be returned.

• number. May be enclosed in "quotation marks".

Can be:

• converted to string using str() function (actually does nothing, as any number is a string)
• multiplied/divided/substracted/added using usual *, /, -, +
• square-rooted using sqrt()
• integer part is returned using int() (ceil/floor/round? - no, what it says – integer part) function

Consequences:

• "2" + 4 == 6;
• "2" == 2;
• But:

  2/0 == "";

• none / "" (empty string)

• none is a constant equal to empty string
• in any number operation on none, it's equal to 0.
• if none is the final evaluation result, it's considered that a property isn't applied at all.

Consequences:

• 2."" == 2;
• 2+"" == 2;
• 2/"" == "";
• none == "";

• boolean

• expression is considered "false" if it equals to "0", "no", "false" or "" (empty string);
• expression is considered "true" otherwise.
• can be turned to a canonical form using boolean() function
• canonical form for "true" is "true", canonical form for "false" is "false"

Consequences:

• boolean("yes") == boolean("true")

Operations

• Comparison operators: ==, <>, !=, <, >, <=, >=. Perform canonization for numbers prior to comparison, so "2" == "02". Implicit canonization doesn't convert numeric value to boolean and vice versa, however.
• String comparison operators: eq, ne: "2" ne "02", "2" eq "2".

Functions

Operators

Operator	Description	Example	JOSM	pgm
==	Equal comparison	"2" == "2" => "true", "3" == "2" => "false", "a" == "a" => "true"	+	+
==	Equal comparison with canonization for numbers	"2" == "02" => "true", "3" == "2" => "false", "a" == "a" => "true"	-	+
!=	Not equal comparison	"2" != "02" => "false", "3" != "2" => "true", "a" != "a" => "false"	+	+
!=	Not equal comparison with canonization for numbers	"2" != "02" => "false", "3" != "2" => "true", "a" != "a" => "false"	-	+
<>	Not equal comparison with canonization for numbers	"2" <> "02" => "false", "3" <> "2" => "true"	+	+
<, <=, >, >=	Numeric greater / lower comparison	"3" > "2" => "true"	+	+
eq	String equality	"2" eq "02" => "false", "3" eq "2" => "false", "a" eq "a" => "true"	-	+
ne	String not equality	"2" ne "02" => "true", "3" ne "2" => "true", "a" ne "a" => "false"	-	+
&&	Logical and	("a" == "a") && ("2" == "3") => "false"	+	+
||	Logical or	("a" == "a") || ("2" == "3") => "true"	+	+
!	Logical not	!("a" == "a") => "false	+	+

General functions

Function	Description	Example	JOSM	pgm
tag(key:string)	returns value of a tag, described by key. In the case there's no such tag, returns none	tag("amenity") => "restaurant"	+	+
prop(key:string)	returns value of a property, described by key. In the case there's no such property, returns none	prop("color") => "#ff0000"	+	+
cond(expr, if, else)	returns if if expr is true, else otherwise.	cond("true", "yes", "no") => "yes"	+	+
any(k1, k2, ...)	returns first k that's not none	any("", "foo", "bar") => "foo"	+	+

"Type check / conversion" functions

Function	Description	Example	JOSM	pgm
num(value)	Returns the parsed value as number or none	num("4.5 ") => 4.5	-	+
str(value)	convert to string (actually does nothing, as any number is a string)	str(4.5) => "4.5"	-	+
boolean(value)	turn value to canonical form; "0", "no", "false", "" => "false"; every other value => "true"	boolean("something") => "true"	-	+

Numeric functions

Function	Description	Example	JOSM	pgm
int(value)	integer part is returned using int() (ceil/floor/round? - no, what it says – integer part) function	int(-5.6) => -5	-	+
max(x1, x2, ...)	returns maximum	max(3, 5, "") => 5	+	+
min(x1, x2, ...)	returns minimum	min(3, 5, "") => 3	+	+
metric(value:string)	Convert parsed (metric) length in map units (pixels)	metric("3m") => 1.5 (when scale is 2m = 1px)	-	+
sqrt(value:float)	returns square root of value	sqrt(4) => 2	+	+

String functions

Function	Description	Example	JOSM	pgm
concat(value, value, ...)	concatenate strings together	concat("amenity: ", tag("amenity")) => "amenity: restaurant"	+	+
value . value . ...	concatenate strings together	"amenity: " . tag("amenity") => "amenity: restaurant"	-	+