Display

print and strings.Format know how to show numbers, strings and booleans. To make one of your own types printable, give it a cast to ustring: an operator cast that returns its text form.

#import "std/strings"

'' A type that can turn itself into text declares a cast to ustring.
type Point
    as float64 x, y
    declare operator cast( ) as ustring
end type

operator Point.cast( ) as ustring
    return strings.Format( "({}, {})", x, y )
end operator

dim p as Point = ( 1.5, -2 )

'' Every place that wants text now accepts a Point.
print p
print "p is " & p
print strings.Format( "p is {} and {:>14}", p, p )

Output

(1.5, -2)
p is (1.5, -2)
p is (1.5, -2) and      (1.5, -2)

Once the cast exists, a Point is accepted wherever text is expected: by print, by the & concatenation operator, and by strings.Format, where the format spec — here {:>14} — applies to the text the cast produced.

Types and operators are covered properly in Custom Types and Objects; this page only shows the hook that makes a value displayable.

Activities

1. Add a z field to Point and include it in the text.
2. Write a Money type holding a number of cents that prints as $12.34.