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Errors, optional, void — mental model

Pedagogical guide (surface S3 + inference B / workspace 0.3.8-dev). Portuguese: errors-null-void.pt-BR.md
Normative: 09-errors, 04-types

Four concepts

Concept	Role	When
void	No useful return	Side-effect functions
optional[T]	Value may be absent	Lookup, EOF — absence is not failure
result[T, E]	Success or failure with reason	I/O, validation
check	Runtime precondition	Invariants

Ori has no null. Use none or err(...).

void

module app.main

import ori.io as io

greet() -> void
    io.println("hello")
end

main()
    greet()
end

optional[T]

module app.main

find_user(id: int) -> optional[string]
    if id == 0
        return none
    end
    return some("alice")
end

main()
    match find_user(1)
        case some(name):
            -- use name
        case none:
    end
end

• Unpack with if some(x) = expr or match.
• try on optional propagates none.
• Postfix ? was removed in S3.

result[T, E]

module app.main

import ori.fs as fs

read_config(path: string) -> result[string, string]
    return fs.read_text(path)
end

• Build with ok(value) / err(reason) (not success / error).
• Handle with match or try expr.

check

divide(a: int, b: int) -> int
    check b != 0, "division by zero"
    return a / b
end

Fails the process on broken contracts; it is not a result.

Quick map

Situation	Use
Print only	-> void
“Not found”, not an error	optional[T]
Failure with message	result[T, string]
Must always be true	check

ori explain name.undefined
ori doctor

Catalog: 13-error-catalog.md.