Conditional expressions for popups, tooltips, and styling

These helpers build GL-style conditional expressions for use anywhere mapgl accepts an expression - most usefully as popup or tooltip content, where they are evaluated against each feature's properties at render time in the browser. That makes display logic possible without precomputing columns, which matters for remote sources (PMTiles, vector tiles) whose attributes cannot be modified from R. They compose with concat(), get_column(), number_format(), match_expr(), and step_expr(); interpolate() also works in popup/tooltip content, but only with numeric outputs (color stops render as an empty string with a console warning - interpolate colors in layer styling, not popups).

Usage

if_else_expr(condition, yes, no = "")

case_expr(..., default)

coalesce_expr(...)

has_column(column)

is_blank(column)

html_escape_expr(expression)

Arguments

condition: For if_else_expr(), an expression that evaluates to TRUE or FALSE for a feature - e.g. is_blank(), has_column(), or a raw comparison like list(">=", get_column("value"), 100).
yes, no: The results when condition is true / false. Strings, numbers, or expressions.
...: For case_expr(), condition/output pairs (condition 1, output 1, condition 2, output 2, ...): the output for the first true condition is used. For coalesce_expr(), values or expressions; the first that evaluates to a non-null value is used (0, FALSE, and "" count as present).
default: For case_expr(), the result when no condition matches. Required (may be NULL).
column: The name of the feature property to test.
expression: For html_escape_expr(), the expression whose result should be HTML-escaped.

Value

A list representing the expression.

Details

Conditional branches are evaluated lazily: only the branch selected for a feature is computed, so e.g. a number_format() in one branch is not evaluated for features that take the other branch.

Popup/tooltip expressions follow the GL expression type system: conditions must evaluate to booleans, math operators require numeric operands (a numeric string from tile data must be converted explicitly, e.g. list("to-number", get_column("value"))), and ordered comparisons require two numbers or two strings. A type error, malformed expression, or non-finite arithmetic result renders as an empty string with a one-time console warning rather than breaking the popup.

is_blank(column) is true when the property is missing, null, or an empty string (""). It does not detect whitespace-only strings or sentinel values like 0 - test those explicitly with a comparison condition.

Note that unlike "{column}" brace templates, expression results are inserted into popups and tooltips as raw HTML (that is what makes concat("<strong>", ...) work). Use these with trusted feature data, or wrap untrusted values in html_escape_expr(), which escapes its result at render time. html_escape_expr() is a mapgl extension for popup/tooltip content only - it is not part of the GL style specification, so don't use it in layer styling or filters.

Examples

# Show a fallback when a property is missing or empty (e.g. parcels from
# PMTiles where some counties don't report addresses or values)
popup_content <- concat(
  "<strong>", get_column("owner_names"), "</strong><br>",
  if_else_expr(
    is_blank("situs_addr"),
    "Address not available",
    get_column("situs_addr")
  ),
  "<br>",
  case_expr(
    list("==", get_column("county"), "HUDSPETH"), "Value not reported",
    default = concat("Total value: $", number_format("total_value"))
  )
)

# First non-missing value
coalesce_expr(get_column("preferred_name"), get_column("name"), "Unnamed")
#> [[1]]
#> [1] "coalesce"
#> 
#> [[2]]
#> [[2]][[1]]
#> [1] "get"
#> 
#> [[2]][[2]]
#> [1] "preferred_name"
#> 
#> 
#> [[3]]
#> [[3]][[1]]
#> [1] "get"
#> 
#> [[3]][[2]]
#> [1] "name"
#> 
#> 
#> [[4]]
#> [1] "Unnamed"
#>