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String formatting with fmt

Problem

You want readable, consistent strings for logs, errors, tables, and user-facing output—without a pile of + concatenation.

Idiom: Use fmt(template, ...args) with {} placeholders

fmt builds a string from a template plus arguments.

  • {} inserts the next argument (auto-indexed).
  • {0}, {1}, … inserts a specific argument (positional).
  • {:<10s}, {:>8}, {:^10s} control alignment and width.
  • {:.2f} controls numeric precision.
  • {:,} adds thousands separators.
  • {:.1%} multiplies by 100 and adds %.
  • \{ and \} let you print literal braces.

Recipe

1) Start simple

var {*} = import("slug.std")

println(fmt("Hello {}", "Slug"))

2) Use positional placeholders when order matters

println(fmt("{1} then {0}", "A", "B"))   // "B then A"

3) Control numeric precision (and percent)

println(fmt("x={:.2f} y={:.2f}", 1.2, 3.4))  // "x=1.20 y=3.40"
println(fmt("success={:.1%}", 0.1234))       // "success=12.3%"

4) Print integers or grouped numbers

println(fmt("id={:d}", 12.9))          // "id=12"
println(fmt("total={:,}", 1234567))    // "total=1,234,567"

5) Make quick fixed-width columns (alignment + width)

println(fmt("|{:>8}|", 12.3))       // right-aligned
println(fmt("|{:<10s}|", "Slug"))   // left-aligned (string)
println(fmt("|{:^10s}|", "Slug"))   // centered (string)

6) Escape braces when you mean literal { / }

println(fmt("value=\\{x\\}"))  // "value={x}"

Notes

  • Auto-indexed ({}) and positional ({0}) placeholders are both supported; prefer {} until you need reordering.
  • Width/alignment is handy for debug output and small CLI tables—keep it simple and it stays readable tomorrow.