php 63 lines · 8 steps

Streaming a fixed-width file into typed objects

A generator reads a fixed-width record file line by line and yields immutable Employee value objects.

Explained by highlit

Intermediate generators value-objects file-parsing immutability lazy-evaluation

1<?php
2 
3declare(strict_types=1);
4 
5final class Employee
6{
  public function __construct(
      public readonly int $id,
      public readonly string $name,
      public readonly string $department,
      public readonly float $salary,
      public readonly \DateTimeImmutable $hiredOn,
  ) {}
14}
15 
16final class FixedWidthEmployeeReader
17{
  private const LAYOUT = [
      'id'         => [0, 6],
      'name'       => [6, 30],
      'department' => [36, 20],
      'salary'     => [56, 12],
      'hiredOn'    => [68, 8],
  ];
25 
  public function read(string $path): \Generator
  {
      $handle = fopen($path, 'rb');
      if ($handle === false) {
          throw new \RuntimeException("Unable to open {$path}");
      }
32 
      try {
          while (($line = fgets($handle)) !== false) {
              $line = rtrim($line, "\r\n");
              if ($line === '') {
                  continue;
              }
              yield $this->parse($line);
          }
      } finally {
          fclose($handle);
      }
  }
45 
  private function parse(string $line): Employee
  {
      $field = static fn (array $spec): string => trim(substr($line, $spec[0], $spec[1]));
49 
      $hiredOn = \DateTimeImmutable::createFromFormat('!Ymd', $field(self::LAYOUT['hiredOn']));
      if ($hiredOn === false) {
          throw new \UnexpectedValueException("Invalid date in record: {$line}");
      }
54 
      return new Employee(
          id: (int) $field(self::LAYOUT['id']),
          name: $field(self::LAYOUT['name']),
          department: $field(self::LAYOUT['department']),
          salary: (float) $field(self::LAYOUT['salary']),
          hiredOn: $hiredOn,
      );
  }
63}

01 / 01

STEP 01

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Three takeaways

1Yielding from a reader keeps memory flat regardless of file size, since records are produced one at a time on demand.
2A declarative column layout separates the file format from the parsing logic, so changes stay in one place.
3Constructor-promoted readonly properties turn a parsed row into a validated, immutable value object in a single expression.

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