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More Formatting of Strings with F$()
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More details on f$()

Most of the text a program builds is a sentence with values in it: a message to a user, a line in a log, a label on a report. Built by joining pieces with plus signs, that sentence comes out inside out — quotes, plus signs, and a str$() around every number:

print 'Her name was ' + name$ + ', she was ' + str$(years) + ' old.'

f$() lets you write the sentence the way you would say it, with each value where it belongs, inside a pair of double brackets. Call each [[]] a slot:

name$ = 'Sally' years = 22 print f$('Her name was [[name$]], she was [[years]] old.') ! Her name was Sally, she was 22 old.

1. What Goes Between the Brackets

Anything you could write on the right side of an =. A variable, some arithmetic, a function call, part of a string (the inner brackets of [[name$[1:3]]] are fine), a whole array or arithmetic on one, which come out as print shows them, and even another f$():

print f$('[[ucase$(name$)]] has [[len(name$)]] letters') ! SALLY has 5 letters print f$('next year: [[years]]') ! next year: 23 print f$('initials: [[name$[1:1]]]') ! initials: S dim scores(3) fill scores with 90, 85, 77 print f$('scores: [[scores]], best [[stats$max(scores)]]') ! scores: 90 85 77, best 90 print f$('doubled: [[scores]], sum [[stats$sum(scores]]') ! doubled: 180 170 154, sum 504

Each slot is worked out when f$() runs, using the variables of the code that calls it. A number comes out the way str$() writes it — no space on either side, unlike print — and a string comes out as it is.

2. Formatting a Number

f$() puts values in place; sprintf$() chooses how a value looks. Use both: put the sprintf$() inside the slot.

total = 1234.5 print f$('Total: [[sprintf$("%.2m",]]') ! Total: 1,234.50 print f$('[[sprintf$("%-10s",]]|') ! Sally | print f$('[[sprintf$("%t@short",]]') ! today's date

3. A Template You Keep

The template is an ordinary string, so it can live in a variable, a constant, or a file, and be reused. The values are read each time f$() runs:

line$ = '[[who$]] scored [[points]]' who$ = 'Ann' \ points = 12 print f$(line$) ! Ann scored 12 who$ = 'Bob' \ points = 15 print f$(line$) ! Bob scored 15

4. When Something Is Wrong

A [[]] causes the exception BADFORMAT. A mistake inside a slot — a misspelled function, an unfinished calculation — causes one too, with the slot's text in the message, and both are ordinary exceptions you can catch. A name that no statement of your program assigns is an error too: [[nmae$]] raises UNKNOWNVAR with the misspelled name in _string$, instead of quietly printing nothing where you expected a name (the same rule applies inside eval()). A variable your program assigns anywhere — even further down — is a variable, as are routine parameters and the built-in names such as _integer. One thing that is not an error: a single bracket is plain text — only the pair opens a slot.

when exception in print f$('Dear [[nmae$]],') use print 'no such variable: '; _string$ ! NMAE$ end when
when exception in print f$('open [[years') use]]

Many languages use curly braces, { }, for this. Sheerpower uses [[]] for three reasons: it is what the language already uses for values in web-page templates ([[variable$]]) and in macros, so it reads as the same idea; it almost never turns up in ordinary text; and curly braces would clash with the JSON that web programs write all day — '{"id": [[id]]}' works as written.

Summary:
  1. Placing (`f$('Her name was [[name$]]')`): Each [[]] slot is worked out when f$() runs and put in as text; numbers as str$() writes them.
  2. Formatting (`[[sprintf$("%.2m",]]`): sprintf$() inside a slot chooses decimals, commas, width and dates.
  3. Reusing: A template is a string; keep it in a variable and the values are read at each call.
  4. Errors: An unclosed [[ and a mistake inside a slot cause exceptions you can catch; an unknown name is a new variable, not an error.
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