Sum type musings

sum_type.php

<?php
declare(strict_types=1);

class UnitOfTemp {
    public const C = 'C';
    public const F = 'F';
    public const K = 'K';

    public const UNITS = [
        self::C,
        self::F,
        self::K,
    ];

    /** @var string */
    private $value;

    public function __construct(string $value) {
        if (! \in_array($value, self::UNITS, true)) {
            throw new \InvalidArgumentException(sprintf('Unit %s is unknown', $value));
        }
        $this->value = $value;
    }

    public function toValue(): string {
        return $this->value;
    }
}

class TempSum1 {
    /** @var float */
    private $value;
    /** @var UnitOfTemp */
    private $unit;

    public function __construct(float $value, UnitOfTemp $unit) {
        $this->value = $value;
        $this->unit = $unit;
    }

    public function match(callable $c, callable $f, callable $k) {
        switch ($this->unit->toValue()) {
            case UnitOfTemp::C: return $c($this->value);
            case UnitOfTemp::F: return $f($this->value);
            case UnitOfTemp::K: return $k($this->value);
        }
    }
}

class TempSum2 {
    /** @var float */
    private $value;
    /** @var string */
    private $unit;

    private const C = 'C';
    private const F = 'F';
    private const K = 'K';

    public static function celsius(float $value): self {
        return new self($value, self::C);
    }

    public static function fahrenheit(float $value): self {
        return new self($value, self::F);
    }

    public static function kelvin(float $value): self {
        return new self($value, self::K);
    }

    public function match(callable $c, callable $f, callable $k) {
        switch ($this->unit) {
            case self::C: return $c($this->value);
            case self::F: return $f($this->value);
            case self::K: return $k($this->value);
        }
    }

    private function __construct(float $value, string $unit) {
        $this->value = $value;
        $this->unit = $unit;
    }
}

// ====

function isFever1(TempSum1 $t): bool {
    return $t->match(
        function ($c) { return $c >= 37;  },
        function ($f) { return $f >= 99;  },
        function ($k) { return $k >= 310; }
    );
}

function isFever2(TempSum2 $t): bool {
    return $t->match(
        function ($c) { return $c >= 37;  },
        function ($f) { return $f >= 99;  },
        function ($k) { return $k >= 310; }
    );
}

assert(false === isFever1(new TempSum1(35,  new UnitOfTemp(UnitOfTemp::C))));
assert(true  === isFever1(new TempSum1(39,  new UnitOfTemp(UnitOfTemp::C))));
assert(false === isFever1(new TempSum1(39,  new UnitOfTemp(UnitOfTemp::F))));
assert(true  === isFever1(new TempSum1(99,  new UnitOfTemp(UnitOfTemp::F))));
assert(false === isFever1(new TempSum1(99,  new UnitOfTemp(UnitOfTemp::K))));
assert(true  === isFever1(new TempSum1(350, new UnitOfTemp(UnitOfTemp::K))));

assert(false === isFever2(TempSum2::celsius(35)));
assert(true  === isFever2(TempSum2::celsius(39)));
assert(false === isFever2(TempSum2::fahrenheit(39)));
assert(true  === isFever2(TempSum2::fahrenheit(99)));
assert(false === isFever2(TempSum2::kelvin(99)));
assert(true  === isFever2(TempSum2::kelvin(350)));