%w error verb

As explained in Go by Example

The %w error verb is a Go formatting directive that records an operand as the wrapped cause of a returned error.

package main

import (
    "fmt"
    "strings"
    . . .
)

type config struct {
    url string
    n   int
    c   int
    rps int
}

func parseArgs(c *config, args []string) error {
    flagSet := map[string]parseFunc{  #1
        "url": stringVar(&c.url),  #2
        "n":   intVar(&c.n),  #3
        "c":   intVar(&c.c),   #3
        "rps": intVar(&c.rps),   #3
    }

    for _, arg := range args {
        name, val, _ := strings.Cut(arg, "=")  #4
        name = strings.TrimPrefix(name, "-")  #5

        setVar, ok := flagSet[name]  #6
        if !ok {
           return fmt.Errorf(
                "flag provided but not defined: -%s",
                name,
            )
        }
        if err := setVar(val); err != nil { #7
            return fmt.Errorf(  #8
                "invalid value %q for flag -%s: %w",  #8
                val, name, err,   #8
            )   #8
        }
    }

    return nil
}

How Go by Example covers it

How deep it goes

Explains wrapping and errors.Is through a worked parser example, without descending into error implementation internals.

Examples

Demonstrates contextual errors from a custom flag parser when strconv.Atoi rejects a flag value.

What you can do after

Diagnose errors with errors.Is while retaining informative command-line parsing context.

How it connects

Connects fmt.Errorf and errors.Is with strconv.Atoi and strconv.ErrSyntax.

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Also explained in

• 

  liveBookmanning.com
  100 Go Mistakes and How to Avoid ThemThe %w directive is a Go fmt formatting directive that records an operand as the underlying error of a returned formatted error.

Compare how 2 books explain %w error verb →

Related concepts

• error wrappingused by · Error wrapping adds contextual information to an error while preserving access to the underlying error for later propagation or inspection.

This explainer was generated by Manning's knowledge graph from the text of Go by Example. The section links point to the parts of the book it draws on.