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How would you design a generic Result type that chains without exceptions?

Build value-or-error storage with variant, exclusive construction, and and_then chaining that forwards the first error unchanged.

TRT practice prompt — not a verified question from a named employer.

The Runtime Theory Team2 min read
Learn it firstProject: A Generic Result Type for the LibraryPro — members onlyBuild a constrained Result for catalogue lookups that carries either a value or an error, composes without exceptions, and reports misuse at compile time.

A strong answer

A strong candidate frames How would you design a generic Result type that chains without exceptions? around the exact memory layout, compiler semantics, and hardware execution model covered in Project: A Generic Result Type for the Library. Build value-or-error storage with variant, exclusive construction, and and_then chaining that forwards the first error unchanged.

Catalogue helpers return records, but records may be absent. Sentinel values conflate valid and invalid data, while exceptions turn common misses into non-local control flow. A generic Result carries either outcome with explicit handling:

One type serves id lookup, title parse and checkout flows. The trace above follows the success path through and_then to the final flag.

In a production interview, always connect the specification rule to its concrete runtime consequence: state what lives on the stack versus the heap or static storage segment, how the compiler or operating system validates alignments, lifetimes, and error codes, and what happens under adversarial or boundary inputs (such as zero-length buffers, integer overflow, concurrent mutation, or partial I/O). Backing your explanation with a small trace of register/memory state transitions immediately separates senior systems engineers from candidates who only memorized syntax.

What the interviewer is listening for

  • Mastery of core invariant: Build a constrained Result class template holding value or error.
  • Mastery of core invariant: Apply map-style composition to chain fallible catalogue queries.
  • Mastery of core invariant: Measure clarity and cost versus exceptions and error codes.
  • Hardware and ABI awareness: Explaining how sizes, alignments, calling conventions, or kernel boundaries behave on real 64-bit targets rather than relying on implementation-defined assumptions.
  • Defensive engineering: Naming the exact compiler flags (-Wall -Wextra -Werror), static analyzers, or runtime sanitizers (-fsanitize=address,undefined,thread) that verify correctness in CI.

Common mistakes

  • Explaining only surface syntax without tracing the underlying memory addresses, ownership lifetime, or kernel/user-space state transitions.
  • Assuming undefined behavior (such as signed overflow, uninitialized reads, strict-aliasing violations, or dangling pointers) will "just work" because it passed an unoptimized -O0 debug build.
  • Ignoring return codes, partial reads/writes, alignment padding, or cleanup paths when an allocation or syscall fails midway through a function.

This answer walks

Practice follow-ups

  1. 01When does Result beat exceptions for frequent catalogue misses?
  2. 02How do you handle mixed error types across chained stages?

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