RFC-3: Variable Interpolation In Strings

In this RFC we propose variable interpolation in strings, which can it easier to show data in UI.

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Status

accepted

Motivation

Say we want to show "Hello, Jack", where Jack is stored in a variable, $name, currently we have to either write a function to concatenate Hello, and $name to form the string, or place two ftd.text nodes, wrapped in a ftd.row. Neither is very nice. So we are proposing variable interpolation, which allows easy generation of such strings with data embedded in it.

Detailed Design

Allow `$var` access in Strings

Any string can now refer to any variable using the $<var-name> syntax, so e.g., we can write Hello, $name, and it will expand into Hello, Jack is $name is Jack.

We already support this if the entire string content was just $<var-name>, we initialise string to it.

Interpolation Creates Formula

In fastn language, formula re-evalutes it's value whenever the underlying variable changes. This means if the variable used in any string interpolation changes, the string will automatically change as well.

`$ curly` syntax

We can also do: The total is $${ count * price }.

Multi line $ curly

-- ftd.text: The total is ${
count * price
}

Lang:

ftd

Escaping interpolation

Sometimes we want to show literally Hello, $name, in this case the author can write Hello, $name, escape the special handling by putting a `` in front of $.

We already do this if the string only contains $<var-name>: $<var-name>