#!/usr/bin/perl

use strict;
use warnings;
use Data::Dumper;

my $EPSILON = 1e-7;

my $f = sub {
  my $x = shift;
  return $x ** 2 - 2 * $x + 7;
};

my $d = sub {
  my ($f, $x) = @_;
    my $d_ = sub {
      my $x = shift;
      return sprintf "%.3f",
          ($f->($x+$EPSILON) - $f->($x-$EPSILON))/( 2 * $EPSILON );
  };
    return defined $x ? $d_->($x) : $d_;
};

print Dumper [ map $f->($_), 0..9 ];
print Dumper [ map $d->($f, $_), 0..9 ];

my $d_of_f = $d->($f);
print Dumper [ map $d_of_f->($_), 0..9 ];