p/moul/vendor/nt/ufmt (v2 preview)

This is ufmt v2, published here before its real path exists. The expected home is gno.land/p/nt/ufmt/v2, which is not published yet. Until it is, import:

1import "gno.land/p/moul/vendor/nt/ufmt/v2"

When gno.land/p/nt/ufmt/v2 goes live, change the import and nothing else. This path is then frozen: it keeps working, it never gets another fix.

Why

p/nt/ufmt/v0 knows a handful of verbs and no flags, and it fails quietly. Sprintf("0x%08X", pc) prints 0x(unhandled verb: %X), Sprintf("%08x", w) prints (unhandled), and Sprintf("%03d", 7) prints 7. r/moul/x/vm/riscvdemo shipped a register dump that was nothing but those.

v2 is Go's fmt, as far as gno can follow it:

flags	+ - # 0
width, precision	%8d, %.2f, %*d, %.*f, %.3s in runes
argument indexes	%[2]d %[1]s, %[1]*[2]d
integers	%b %c %d %o %O %q %x %X %U, every int and uint type
floats	%b %e %E %f %F %g %G %x %X, float32 and float64
strings, []byte	%s %q %x %X, % x, %#q
methods	Formatter, GoStringer, error, Stringer, in Go's order
errors	Errorf with %w, one or several, seen by errors.Is
composites	slices of the basic types, []any, map[string]T sorted by key

A mistake is printed, never panicked, with Go's strings: %!d(string=hi), %!d(MISSING), %!(EXTRA int=1), %!(NOVERB). A Render cannot be taken down by a format string.

How it is tested

gofmt_test.gno is 210 cases whose expected output was produced by running each one through Go's fmt.Sprintf (go1.27.1), so a failure there is, by construction, a place v2 and Go disagree. It also costs less gas than v0 on the calls realms make most: about 211k against 254k per Sprintf("id %d is %s (%t)", ...), measured with gno test.

What it cannot do

gno has no reflect. A struct, a pointer, or a named type without a String method prints (unhandled), and %T names only the types it knows. Give the type a String method. %p is not supported.

Migrating from v0

Mostly a change of import, with these differences, all of them toward Go:

• Sprint adds a space only between two operands that are both non-strings.
• a float prints in its shortest form from %v and Sprint (3.14, not 3.140000), and %e defaults to 6 digits.
• %v of a []byte is [104 105]; use %s for the text.
• a missing or extra argument is written into the output instead of panicking.
• error wins over String when a value has both.

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Part of moul/gno-contracts — moul's versioned gno.land contracts. See the repository for the full catalog, build/test tooling, and usage.

⚠️ Disclaimer: provided as-is, without warranty; not security-audited. Full disclaimer: DISCLAIMER.