sickn33/agentic-awesome-skills · error · Error

Age cannot be negative

Error message

Age cannot be negative

What it means

Second guard in the same fp-pragmatic parseAge example (skills/fp-pragmatic/SKILL.md:114): thrown when the input parses to a number but that number is negative, i.e. a semantically invalid age that survives the NaN check.

Source

Thrown at skills/fp-pragmatic/SKILL.md:114

**Plain language translation:**
- `O.fromNullable(x)` = "wrap this value, treating null/undefined as 'nothing'"
- `O.flatMap(fn)` = "if we have something, apply this function"
- `O.getOrElse(() => default)` = "unwrap, or use this default if nothing"

### 3. Either: Make Errors Explicit

Stop throwing exceptions for expected failures. Return errors as values.

```typescript
import * as E from 'fp-ts/Either'
import { pipe } from 'fp-ts/function'

// Before: Hidden failure mode
function parseAge(input: string): number {
  const age = parseInt(input, 10)
  if (isNaN(age)) throw new Error('Invalid age')
  if (age < 0) throw new Error('Age cannot be negative')
  return age
}

// After: Errors are visible in the type
function parseAge(input: string): E.Either<string, number> {
  const age = parseInt(input, 10)
  if (isNaN(age)) return E.left('Invalid age')
  if (age < 0) return E.left('Age cannot be negative')
  return E.right(age)
}

// Using it
const result = parseAge(userInput)
if (E.isRight(result)) {
  console.log(`Age is ${result.right}`)
} else {
  console.log(`Error: ${result.left}`)
}

View on GitHub (pinned to 58d857988f)

Solutions

1. Add domain constraints to the Either version: chain a non-negative check that returns E.left with a cannot-be-negative message
2. Constrain at the input layer: number input with min=0 in forms, schema validation with a nonnegative rule
3. Audit data sources if negatives appear unexpectedly (sign flips, unit confusion)

Example fix

// before
if (age < 0) throw new Error('Age cannot be negative')
return age

// after
return age < 0
  ? E.left('Age cannot be negative')
  : E.right(age)

const age = Number(input)
if (!Number.isInteger(age) || age < 0) {
  return badRequest('age must be a non-negative integer')
}

const isNonNegativeInt = (n: unknown): n is number =>
  typeof n === 'number' && Number.isInteger(n) && n >= 0

When it happens

Trigger: parseAge with -5 or any input whose parsed integer is below zero; also decimal strings that parseInt truncates but that still yield a value of -1 or less.

Common situations: Users typing a minus sign or negative numbers in age fields; sign errors in upstream data feeds; test fixtures using sentinel negative values.

Related errors

• Invalid range "${t}" in ${fieldDef.name}
• Expected 5 fields (got ${parts.length}). Format: minute hour
• Invalid age
• Invalid JSON: ${error}
• Value ${v} out of range for ${fieldDef.name} (${fieldDef.min

AI-assisted analysis of sickn33/agentic-awesome-skills@58d857988f (2026-08-26). Data as JSON: /api/errors/67002ab60c657c9d. Report an issue: GitHub.