Principle:LaurentMazare Tch rs Error Handling

Knowledge Sources	LaurentMazare_Tch_rs
Domains	Software Engineering, Error Handling, Type Systems
Last Updated	2026-02-08 00:00 GMT

Overview

Typed error handling uses algebraic data types to represent distinct failure modes, enabling exhaustive pattern matching and structured error propagation through computation chains.

Description

Typed error handling replaces opaque error codes or unchecked exceptions with sum types (tagged unions or enums) whose variants each represent a specific failure mode. This approach leverages the type system to ensure that error conditions are explicitly handled at compile time.

An error enum typically defines variants for each category of failure, such as:

Shape mismatches -- Operations on tensors with incompatible dimensions
Internal library errors -- Failures originating from the underlying computational engine
I/O errors -- File system or network failures during data loading or model serialization
Conversion errors -- Type or format mismatches when converting between representations
Configuration errors -- Invalid parameters or missing required settings

The Result type wraps either a successful value or an error, forcing the caller to explicitly handle both outcomes. This eliminates silent failures and makes error propagation visible in function signatures. The question-mark operator provides ergonomic syntax for propagating errors up the call stack, automatically converting between compatible error types.

Context propagation enriches errors with additional information (e.g., which operation failed, what shapes were involved) as they bubble up through layers of abstraction. This creates an error chain that aids debugging by showing the full path from root cause to the point where the error is reported.

Usage

Apply typed error handling when:

Functions can fail in multiple distinct ways that callers may handle differently
Error conditions should be enforced at compile time rather than discovered at runtime
Debugging requires knowing the chain of operations that led to a failure
Library APIs need to communicate failure causes clearly to downstream consumers

Theoretical Basis

Algebraic Error Types

An error type is defined as a sum type with $n$ variants:

$E = E_{1} | E_{2} | \dots | E_{n}$

Each variant may carry additional data describing the specific failure. A function that can fail returns:

$f : A \to Result (B, E)$

Exhaustive Pattern Matching

The type system ensures every variant is handled:

match result:
  Ok(value)      -> use value
  Err(variant_1) -> handle case 1
  Err(variant_2) -> handle case 2
  ...

Adding a new error variant causes a compile-time error at every unhandled match site, preventing regressions.

Error Propagation

Errors compose through monadic chaining. Given functions $f : A \to Result (B, E)$ and $g : B \to Result (C, E)$ :

$(f then g) (a) = {\begin{cases} g (b) & if f (a) = Ok (b) \\ Err (e) & if f (a) = Err (e) \end{cases}$

Error Conversion

When different subsystems define their own error types, a From conversion allows automatic wrapping:

$From : E_{sub} \to E_{parent}$

This enables composing operations across subsystem boundaries without manual conversion at each call site.

Related Pages

Implementation:LaurentMazare_Tch_rs_TchError

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