Learning Curl: {value expr} and Curl as an expression-based language

One thing that I have noticed in C# and Java programmers learning Curl is that if they rely on one of the available books (and even to some extent if they use the on-line Curl resources) there is a puzzle about {value some-expression} code blocks.

Here is an example from a book on Curl:

{value 2fortnights + 1day}

This an example of a Curl expression in which quantities are added (note the optional use of the plural for the user-defined quantity.)

{curl 7.0 applet}
{let public constant fortnight:Type = {type-of 14days}}
time in seconds is {value 2fortnights + 1day}

 
In keeping with the Gentle Slope approach to reduce the steepness of the Curl learning-curve, the text

time in seconds is

appears in the browser as text.
We say that this bit of text is at the top-level.
What follows, however, is an expression within curly braces and what we want to display is the result of that addition of the two Time quantities. We could have written this expression using optional parentheses as

{value (2fortnights + 1day) }

The {value } macro has been used to return a value: that is all.
To show that this is so, consider this alternative:

{String (2fortnights + 1day) }

The default constructor for the String class also returns a value (an instance of a String.)
Here is another alternative using assignment to a variable named tos:

{let tos:Time = 2fortnights + 1day}
The Time value held in the variable is {String tos}

And finally a complete alternative applet:

{curl 7.0 applet}
{let public constant fortnight:Type = {type-of 14days}}
{let tos:Time = 2fortnights + 1day}

The time in seconds equals {value 2fortnights + 1day}

Time quantity in the variable is {String tos}

Number of days: {format "%.0f", (tos / 1day)}

which displays in the browser as follows:

The time in seconds equals 2.5056e+006s

Time quantity in the variable is 2.5056e+006s

Number of days: 29

A more compact version of the applet using explicit {br} line formatting rather than relying on Curl to display the extra new lines might have been:

{curl 7.0 applet}
{let public constant fortnight:Type = {type-of 14days}}
{let tos:Time = 2fortnights + 1day}
The time in seconds equals {value 2fortnights + 1day} {br}
Time quantity in the variable is {String tos} {br}
Number of days: {format "%.0f", (tos / 1day)}

And so we proceed to wend our way up the 'Gentle Slope' of Curl.