Exceptions

Recover with Default

A catch block can choose a default value and let the program continue.

fallback Recovery usually means producing a clear fallback value, not hiding the failure.

Recover with Default

raw
recover_default.php
Replay: real traced execution (multi-file project)
<?php
function parseCount(string $raw): int
{
    if (!ctype_digit($raw)) {
        throw new Exception("not a number");
    }

    return (int) $raw;
}

$raw = "12";

try {
    $count = parseCount($raw);
    $source = "input";
} catch (Exception $error) {
    $count = 0;
    $source = "default";
}

echo "raw=" . $raw . "\n";
echo "count=" . $count . "\n";
echo "source=" . $source . "\n";

<?php
function parseCount(string $raw): int
{
    if (!ctype_digit($raw)) {
        throw new Exception("not a number");
    }

    return (int) $raw;
}

$raw = "bad";

try {
    $count = parseCount($raw);
    $source = "input";
} catch (Exception $error) {
    $count = 0;
    $source = "default";
}

echo "raw=" . $raw . "\n";
echo "count=" . $count . "\n";
echo "source=" . $source . "\n";

<?php
function parseCount(string $raw): int
{
    if (!ctype_digit($raw)) {
        throw new Exception("not a number");
    }

    return (int) $raw;
}

$raw = "5";

try {
    $count = parseCount($raw);
    $source = "input";
} catch (Exception $error) {
    $count = 0;
    $source = "default";
}

echo "raw=" . $raw . "\n";
echo "count=" . $count . "\n";
echo "source=" . $source . "\n";

  1. $raw ← 12

    11$raw→ 12 = "12"; //@raw="bad", "5"1213try {14    $count = parseCount($raw12);15    $source = "input";

  2. function parseCount(string $raw): int

    1<?php2function parseCount(string $raw12): int3{4    if (!ctype_digit($raw)) {5        throw new Exception("not a number");6    }78    return (int) $raw12;9}

  3. $count ← 12, $source ← input

    13try {14    $count→ 12 = parseCount($raw12);15    $source→ input = "input";16} catch (Exception $error) {17    $count = 0;18    $source = "default";19}2021echo "raw=" . $raw12 . "\n";22echo "count=" . $count12 . "\n";23echo "source=" . $sourceinput . "\n";
    outputraw=12
count=12
source=input

  1. $raw ← bad

    11$raw→ bad = "bad";1213try {14    $count = parseCount($rawbad);15    $source = "input";

  2. function parseCount(string $raw): int

    1<?php2function parseCount(string $rawbad): int3{4    if (!ctype_digit($raw)) {

  3. if (!ctype_digit($raw))

    3{4    if (!ctype_digit($rawbad)) {5        throw new Exception("not a number");6    }

  4. echo "raw=" . $raw . " ";

    16} catch (Exception $error) {17    $count = 0;18    $source = "default";19}2021echo "raw=" . $rawbad . "\n";22echo "count=" . $count0 . "\n";23echo "source=" . $sourcedefault . "\n";
    outputraw=bad
count=0
source=default

  1. $raw ← 5

    11$raw→ 5 = "5";1213try {14    $count = parseCount($raw5);15    $source = "input";

  2. function parseCount(string $raw): int

    1<?php2function parseCount(string $raw5): int3{4    if (!ctype_digit($raw)) {5        throw new Exception("not a number");6    }78    return (int) $raw5;9}

  3. $count ← 5, $source ← input

    13try {14    $count→ 5 = parseCount($raw5);15    $source→ input = "input";16} catch (Exception $error) {17    $count = 0;18    $source = "default";19}2021echo "raw=" . $raw5 . "\n";22echo "count=" . $count5 . "\n";23echo "source=" . $sourceinput . "\n";
    outputraw=5
count=5
source=input