(use-modules (hashing sha-2)
             (rnrs bytevectors)
             (ice-9 binary-ports)
             (ice-9 match)
             (ice-9 time))

(define (port-sha256 port)
  ;; Return the SHA256 of the data read from PORT.
  (define bv (make-bytevector 65536))
  (define hash (make-sha-256))

  (let loop ()
    (match (get-bytevector-n! port bv 0
                              (bytevector-length bv))
      ((? eof-object?)
       (sha-256-finish! hash)
       hash)
      (n
       (sha-256-update! hash bv 0 n)
       (loop)))))

(define (file-sha256 file)
  ;; Return the SHA256 of FILE.
  (call-with-input-file file port-sha256))

(time (pk 'hash (sha-256->string (file-sha256 "/gnu/store/zfp7d4wr5hbl5lrnzs8c15bsc3ygq74g-guile-2.2.6.tar.xz"))))