Errors and catching

Faber does not throw errors the way most languages do. A failure travels back through a dedicated error channel, declared in the signature and separate from the return value. That one decision shapes everything on this page.

The error channel#

→ declares what a function returns. ⇥ declares what it may fail with.

reader locale

faber format --locale en — English reader surface

fn divide(int a, int b) → int ⇥ string {
    if b ≡ 0 {
        throw "division by zero"
    }
    return a / b
}

faber format --locale la — canonical Faber

functio divide(numerus a, numerus b) → numerus ⇥ textus {
    si b ≡ 0 {
        iace "division by zero"
    }
    redde a / b
}

faber format --locale th-TH — Thai

ฟังก์ชัน divide(จำนวน a, จำนวน b) → จำนวน ⇥ ข้อความ {
    ถ้า b ≡ 0 {
        โยน "division by zero"
    }
    คืน a / b
}

faber format --locale zh-Hans — Simplified Chinese

函数 divide(整数 a, 整数 b) → 整数 ⇥ 文本 {
    如果 b ≡ 0 {
        抛错 "division by zero"
    }
    返回 a / b
}

faber format --locale zh-Hant — Traditional Chinese

函式 divide(整數 a, 整數 b) → 整數 ⇥ 文字 {
    若 b ≡ 0 {
        拋出 "division by zero"
    }
    傳回 a / b
}

faber format --locale vi — Vietnamese

hàm divide(số a, số b) → số ⇥ văn_bản {
    nếu b ≡ 0 {
        ném "division by zero"
    }
    trả a / b
}

faber format --locale ar — Arabic

دالة divide(عدد a, عدد b) → عدد ⇥ نص {
    إذا b ≡ 0 {
        ارم "division by zero"
    }
    أعد a / b
}

faber format --locale hi — Hindi

फलन divide(संख्या a, संख्या b) → संख्या ⇥ पाठ {
    यदि b ≡ 0 {
        इधरफेंको "division by zero"
    }
    लौटाओ a / b
}

Read it as: returns a number, or fails with a textus. The two channels are independent — the success type is not widened, wrapped, or made optional to accommodate failure. A caller reading the signature knows both shapes before looking at the body.

iace sends a value down the error channel. It is not return: it exits by the other route.

Catching#

catch binds whatever came down the error channel.

reader locale

faber format --locale en — English reader surface

fn divide(int a, int b) → int ⇥ string {
    if b ≡ 0 {
        throw "division by zero"
    }
    return a / b
}

fn tutum(int a, int b) → int {
    do {
        return divide(a, b)
    }
    catch err {
        warn err
        return 0
    }
}

main {
    print tutum(10, 2)
    print tutum(10, 0)
}

faber format --locale la — canonical Faber

functio divide(numerus a, numerus b) → numerus ⇥ textus {
    si b ≡ 0 {
        iace "division by zero"
    }
    redde a / b
}

functio tutum(numerus a, numerus b) → numerus {
    fac {
        redde divide(a, b)
    }
    cape err {
        mone err
        redde 0
    }
}

incipit {
    nota tutum(10, 2)
    nota tutum(10, 0)
}

faber format --locale th-TH — Thai

ฟังก์ชัน divide(จำนวน a, จำนวน b) → จำนวน ⇥ ข้อความ {
    ถ้า b ≡ 0 {
        โยน "division by zero"
    }
    คืน a / b
}

ฟังก์ชัน tutum(จำนวน a, จำนวน b) → จำนวน {
    ทำ {
        คืน divide(a, b)
    }
    จับ err {
        เตือน err
        คืน 0
    }
}

เริ่ม {
    บันทึก tutum(10, 2)
    บันทึก tutum(10, 0)
}

faber format --locale zh-Hans — Simplified Chinese

函数 divide(整数 a, 整数 b) → 整数 ⇥ 文本 {
    如果 b ≡ 0 {
        抛错 "division by zero"
    }
    返回 a / b
}

函数 tutum(整数 a, 整数 b) → 整数 {
    执行 {
        返回 divide(a, b)
    }
    捕获 err {
        警告 err
        返回 0
    }
}

入口 {
    显示 tutum(10, 2)
    显示 tutum(10, 0)
}

faber format --locale zh-Hant — Traditional Chinese

函式 divide(整數 a, 整數 b) → 整數 ⇥ 文字 {
    若 b ≡ 0 {
        拋出 "division by zero"
    }
    傳回 a / b
}

函式 tutum(整數 a, 整數 b) → 整數 {
    執行 {
        傳回 divide(a, b)
    }
    捕捉 err {
        警告 err
        傳回 0
    }
}

入口 {
    註記 tutum(10, 2)
    註記 tutum(10, 0)
}

faber format --locale vi — Vietnamese

hàm divide(số a, số b) → số ⇥ văn_bản {
    nếu b ≡ 0 {
        ném "division by zero"
    }
    trả a / b
}

hàm tutum(số a, số b) → số {
    làm {
        trả divide(a, b)
    }
    bắt err {
        cảnh_báo err
        trả 0
    }
}

bắt_đầu {
    ghi_chú tutum(10, 2)
    ghi_chú tutum(10, 0)
}

faber format --locale ar — Arabic

دالة divide(عدد a, عدد b) → عدد ⇥ نص {
    إذا b ≡ 0 {
        ارم "division by zero"
    }
    أعد a / b
}

دالة tutum(عدد a, عدد b) → عدد {
    افعل {
        أعد divide(a, b)
    }
    التقط err {
        نبه err
        أعد 0
    }
}

بداية {
    اعرض tutum(10, 2)
    اعرض tutum(10, 0)
}

faber format --locale hi — Hindi

फलन divide(संख्या a, संख्या b) → संख्या ⇥ पाठ {
    यदि b ≡ 0 {
        इधरफेंको "division by zero"
    }
    लौटाओ a / b
}

फलन tutum(संख्या a, संख्या b) → संख्या {
    करो {
        लौटाओ divide(a, b)
    }
    पकड़ो err {
        चेताओ err
        लौटाओ 0
    }
}

आरंभ {
    दिखाओ tutum(10, 2)
    दिखाओ tutum(10, 0)
}

tutum has no ⇥ in its signature, and that is the point: it handles the failure, so it cannot itself fail. The channel stops here.

`catch` attaches to more than `do`#

This is the part that surprises people arriving from other languages. catch is not the tail of a try statement — it attaches to structured statements generally. A loop can carry its own handler:

reader locale

faber format --locale en — English reader surface

main {
    const list<string> raw ← ["1", "two", "3"]
    do {
        for from raw const item {
            const int n ← item ↦ int
            print n
        }
    }
    catch err {
        warn err
    }
}

faber format --locale la — canonical Faber

incipit {
    fixum lista<textus> raw ← ["1", "two", "3"]
    fac {
        itera ex raw fixum item {
            fixum numerus n ← item ↦ numerus
            nota n
        }
    }
    cape err {
        mone err
    }
}

faber format --locale th-TH — Thai

เริ่ม {
    คงที่ รายการ<ข้อความ> raw ← ["1", "two", "3"]
    ทำ {
        วน ออก raw คงที่ item {
            คงที่ จำนวน n ← item ↦ จำนวน
            บันทึก n
        }
    }
    จับ err {
        เตือน err
    }
}

faber format --locale zh-Hans — Simplified Chinese

入口 {
    常量 列表<文本> raw ← ["1", "two", "3"]
    执行 {
        遍历 取自 raw 常量 item {
            常量 整数 n ← item ↦ 整数
            显示 n
        }
    }
    捕获 err {
        警告 err
    }
}

faber format --locale zh-Hant — Traditional Chinese

入口 {
    定值 列表<文字> raw ← ["1", "two", "3"]
    執行 {
        遍歷 取自 raw 定值 item {
            定值 整數 n ← item ↦ 整數
            註記 n
        }
    }
    捕捉 err {
        警告 err
    }
}

faber format --locale vi — Vietnamese

bắt_đầu {
    hằng danh_sách<văn_bản> raw ← ["1", "two", "3"]
    làm {
        lặp từ raw hằng item {
            hằng số n ← item ↦ số
            ghi_chú n
        }
    }
    bắt err {
        cảnh_báo err
    }
}

faber format --locale ar — Arabic

بداية {
    ثابت قائمة<نص> raw ← ["1", "two", "3"]
    افعل {
        كرر من raw ثابت item {
            ثابت عدد n ← item ↦ عدد
            اعرض n
        }
    }
    التقط err {
        نبه err
    }
}

faber format --locale hi — Hindi

आरंभ {
    स्थिर सूची<पाठ> raw ← ["1", "two", "3"]
    करो {
        दोहराओ सेवन raw स्थिर item {
            स्थिर संख्या n ← item ↦ संख्या
            दिखाओ n
        }
    }
    पकड़ो err {
        चेताओ err
    }
}

And so can a while loop:

reader locale

faber format --locale en — English reader surface

main {
    var int i ← 0
    do {
        while i ≺ 3 {
            i ← i + 1
            if i ≡ 2 {
                throw "halfway"
            }
        }
    }
    catch err {
        warn err
    }
}

faber format --locale la — canonical Faber

incipit {
    varia numerus i ← 0
    fac {
        dum i ≺ 3 {
            i ← i + 1
            si i ≡ 2 {
                iace "halfway"
            }
        }
    }
    cape err {
        mone err
    }
}

faber format --locale th-TH — Thai

เริ่ม {
    แปร จำนวน i ← 0
    ทำ {
        ขณะ i ≺ 3 {
            i ← i + 1
            ถ้า i ≡ 2 {
                โยน "halfway"
            }
        }
    }
    จับ err {
        เตือน err
    }
}

faber format --locale zh-Hans — Simplified Chinese

入口 {
    变量 整数 i ← 0
    执行 {
        当 i ≺ 3 {
            i ← i + 1
            如果 i ≡ 2 {
                抛错 "halfway"
            }
        }
    }
    捕获 err {
        警告 err
    }
}

faber format --locale zh-Hant — Traditional Chinese

入口 {
    變值 整數 i ← 0
    執行 {
        當 i ≺ 3 {
            i ← i + 1
            若 i ≡ 2 {
                拋出 "halfway"
            }
        }
    }
    捕捉 err {
        警告 err
    }
}

faber format --locale vi — Vietnamese

bắt_đầu {
    biến số i ← 0
    làm {
        trong_khi i ≺ 3 {
            i ← i + 1
            nếu i ≡ 2 {
                ném "halfway"
            }
        }
    }
    bắt err {
        cảnh_báo err
    }
}

faber format --locale ar — Arabic

بداية {
    متغير عدد i ← 0
    افعل {
        طالما i ≺ 3 {
            i ← i + 1
            إذا i ≡ 2 {
                ارم "halfway"
            }
        }
    }
    التقط err {
        نبه err
    }
}

faber format --locale hi — Hindi

आरंभ {
    चर संख्या i ← 0
    करो {
        जबतक i ≺ 3 {
            i ← i + 1
            यदि i ≡ 2 {
                इधरफेंको "halfway"
            }
        }
    }
    पकड़ो err {
        चेताओ err
    }
}

The handler belongs to the construct it follows. There is no separate try keyword to wrap things in — do is simply the bare block form for when you want a handler and have no other statement to hang it on.

Failing without a message#

A function that declares ⇥ and does not catch propagates automatically — callers must deal with it or declare their own channel.

reader locale

faber format --locale en — English reader surface

fn parse(string raw) → int ⇥ string {
    if raw ≡ "" {
        throw "empty input"
    }
    return raw ↦ int ⇥ 0
}

fn duplum(string raw) → int ⇥ string {
    return parse(raw) * 2
}

main {
    do {
        print duplum("21")
    }
    catch err {
        warn err
    }
}

faber format --locale la — canonical Faber

functio parse(textus raw) → numerus ⇥ textus {
    si raw ≡ "" {
        iace "empty input"
    }
    redde raw ↦ numerus ⇥ 0
}

functio duplum(textus raw) → numerus ⇥ textus {
    redde parse(raw) * 2
}

incipit {
    fac {
        nota duplum("21")
    }
    cape err {
        mone err
    }
}

faber format --locale th-TH — Thai

ฟังก์ชัน parse(ข้อความ raw) → จำนวน ⇥ ข้อความ {
    ถ้า raw ≡ "" {
        โยน "empty input"
    }
    คืน raw ↦ จำนวน ⇥ 0
}

ฟังก์ชัน duplum(ข้อความ raw) → จำนวน ⇥ ข้อความ {
    คืน parse(raw) * 2
}

เริ่ม {
    ทำ {
        บันทึก duplum("21")
    }
    จับ err {
        เตือน err
    }
}

faber format --locale zh-Hans — Simplified Chinese

函数 parse(文本 raw) → 整数 ⇥ 文本 {
    如果 raw ≡ "" {
        抛错 "empty input"
    }
    返回 raw ↦ 整数 ⇥ 0
}

函数 duplum(文本 raw) → 整数 ⇥ 文本 {
    返回 parse(raw) * 2
}

入口 {
    执行 {
        显示 duplum("21")
    }
    捕获 err {
        警告 err
    }
}

faber format --locale zh-Hant — Traditional Chinese

函式 parse(文字 raw) → 整數 ⇥ 文字 {
    若 raw ≡ "" {
        拋出 "empty input"
    }
    傳回 raw ↦ 整數 ⇥ 0
}

函式 duplum(文字 raw) → 整數 ⇥ 文字 {
    傳回 parse(raw) * 2
}

入口 {
    執行 {
        註記 duplum("21")
    }
    捕捉 err {
        警告 err
    }
}

faber format --locale vi — Vietnamese

hàm parse(văn_bản raw) → số ⇥ văn_bản {
    nếu raw ≡ "" {
        ném "empty input"
    }
    trả raw ↦ số ⇥ 0
}

hàm duplum(văn_bản raw) → số ⇥ văn_bản {
    trả parse(raw) * 2
}

bắt_đầu {
    làm {
        ghi_chú duplum("21")
    }
    bắt err {
        cảnh_báo err
    }
}

faber format --locale ar — Arabic

دالة parse(نص raw) → عدد ⇥ نص {
    إذا raw ≡ "" {
        ارم "empty input"
    }
    أعد raw ↦ عدد ⇥ 0
}

دالة duplum(نص raw) → عدد ⇥ نص {
    أعد parse(raw) * 2
}

بداية {
    افعل {
        اعرض duplum("21")
    }
    التقط err {
        نبه err
    }
}

faber format --locale hi — Hindi

फलन parse(पाठ raw) → संख्या ⇥ पाठ {
    यदि raw ≡ "" {
        इधरफेंको "empty input"
    }
    लौटाओ raw ↦ संख्या ⇥ 0
}

फलन duplum(पाठ raw) → संख्या ⇥ पाठ {
    लौटाओ parse(raw) * 2
}

आरंभ {
    करो {
        दिखाओ duplum("21")
    }
    पकड़ो err {
        चेताओ err
    }
}

duplum never mentions failure in its body. It declares the channel and lets parse's failure travel through.

Errors as a parameter#

errata marks a parameter that carries an error value, for functions written to receive one rather than produce it.

reader locale

faber format --locale en — English reader surface

fn logga(string context) → void {
    warn context
}

faber format --locale la — canonical Faber

functio logga(textus context) → vacuum {
    mone context
}

faber format --locale th-TH — Thai

ฟังก์ชัน logga(ข้อความ context) → เปล่า {
    เตือน context
}

faber format --locale zh-Hans — Simplified Chinese

函数 logga(文本 context) → 无值 {
    警告 context
}

faber format --locale zh-Hant — Traditional Chinese

函式 logga(文字 context) → 空值 {
    警告 context
}

faber format --locale vi — Vietnamese

hàm logga(văn_bản context) → trống {
    cảnh_báo context
}

faber format --locale ar — Arabic

دالة logga(نص context) → فراغ {
    نبه context
}

faber format --locale hi — Hindi

फलन logga(पाठ context) → रिक्त {
    चेताओ context
}

Related: Testing · Conversions for ⇥ in its other role, as conversion recovery · Errors and testing for the full treatment.

Errors and catching

The error channel#

Catching#

catch attaches to more than do#

Failing without a message#

Errors as a parameter#

`catch` attaches to more than `do`#