(use-modules (ice-9 rdelim))

(define (file-lines-match? file prefix)
  (with-input-from-file file
    (lambda ()
      (catch 'found
        (lambda ()
          (let loop ((line (read-line)))
            (cond
             ((eof-object? line) #false)
             ((string-prefix? prefix line)
              (throw 'found))
             (else (loop (read-line))))))
        (const #true)))))

Generated by Ricardo Wurmus using scpaste at Fri Feb 5 09:52:43 2021. CET. (original)