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Reflection — Hands-on Tasks

Work through these in order. Use Go 1.22+.

Task 1: Print any value

Write a function dump(v any) that prints the type and value, working for any input.

Acceptance criteria - [ ] Handles ints, strings, slices, maps, and structs. - [ ] Recursively prints struct fields with name=value. - [ ] Doesn't panic on nil input.

Task 2: Settability practice

type User struct {
    Name string
    Age  int
}

func set(u User, name string, age int) { /* try this */ }
func setPtr(u *User, name string, age int) { /* and this */ }

Acceptance criteria - [ ] You demonstrate that set cannot modify the struct. - [ ] setPtr uses reflection to mutate Name and Age through the pointer. - [ ] You explain in one sentence why one works and the other doesn't.

Task 3: Env loader

Build a function LoadEnv(target any) that:

  • Takes a pointer to a struct.
  • For each exported field, reads the env:"NAME" tag.
  • Looks up the env var and sets the field if present.
  • Supports string, int, and bool field types.

Acceptance criteria - [ ] Works for a struct with at least one field of each supported type. - [ ] Skips fields without the env tag. - [ ] Returns an error for fields with the tag but a type it can't decode.

Task 4: Struct walker

Build a Walk(v any, fn func(name string, value any)) that calls fn for every (exported) field of a struct, recursing into nested structs.

Acceptance criteria - [ ] Handles a struct with at least one nested struct. - [ ] Reports the dotted name (Outer.Inner.Field). - [ ] Skips unexported fields.

Task 5: Plan-cached decoder

Build a string→struct decoder. Inputs:

  • A pointer to a struct with field tags key:"foo".
  • A map[string]string.

Acceptance criteria - [ ] On first call for a type, the function reflects and builds a plan. - [ ] On subsequent calls for the same type, it reuses the cached plan (verify with a counter). - [ ] Bench shows the second call is ≥5× faster than the first.

Task 6: Method dispatch table

Given an interface with three methods (Create, Read, Delete) implemented by a struct, dispatch based on a string name read at runtime.

Acceptance criteria - [ ] Uses MethodByName to find the right method. - [ ] Returns an error if the name doesn't match. - [ ] You then rewrite using a map[string]func(...) and bench the difference.

Task 7: DeepEqual vs go-cmp

a := []int{}
b := []int(nil)

Acceptance criteria - [ ] You confirm reflect.DeepEqual(a, b) is false. - [ ] You confirm cmp.Equal(a, b, cmpopts.EquateEmpty()) is true. - [ ] You write a one-paragraph note for the team explaining when to use which.

Task 8: Tag parser

Build a parser for a complex tag string: name,omitempty,default=X,validate=email.

Acceptance criteria - [ ] Parses the comma-separated parts into a struct. - [ ] Handles key=value correctly (e.g., default=X). - [ ] Has at least 5 unit tests including edge cases.

Task 9: MakeFunc wrapper

Wrap any function with logging:

func Wrap(orig any) any { /* ... */ }

Acceptance criteria - [ ] Returned function has the same signature as the original. - [ ] Logs "calling" and "returned" around each call. - [ ] Works for at least two different function signatures.

Task 10: Implement a sub-Stringer

Write a function String(v any) string that:

  • If v implements fmt.Stringer, calls String().
  • Else, uses reflection to render the value generically.

Acceptance criteria - [ ] Type assertion path runs first (fast). - [ ] Reflection path runs only when needed. - [ ] Bench shows the assertion path is much faster.

Task 11: Build an offset table

Given a struct type, build a slice of (name, offset, kind) for every exported field.

Acceptance criteria - [ ] Output includes offsets in bytes. - [ ] You verify offsets by allocating a struct and pointing at a field via unsafe.Add. - [ ] You explain how this enables zero-reflection field access at runtime.

Task 12: Replace reflection with generics

Pick a small helper that uses reflection (e.g., Map(s []any, fn func(any) any) []any). Rewrite it using generics.

Acceptance criteria - [ ] Type-safe at compile time. - [ ] No reflection. - [ ] Bench shows zero allocations.

Stretch — Task 13: A toy JSON encoder

Write encode(v any) ([]byte, error) that supports:

  • Booleans, numbers, strings.
  • Slices and maps (top-level only is fine).
  • Structs with exported fields.

Acceptance criteria - [ ] Produces valid JSON for the supported types. - [ ] Plans per type are cached. - [ ] Bench against encoding/json and report the ratio. - [ ] Honors json:"name,omitempty" tags.

Submission

For each task: code, the test or bench output, and 2–3 lines explaining the design choice. These artifacts demonstrate not just understanding but the ability to apply reflection responsibly.