Can you attack with a creature the turn an equipment that grants haste is attached to it?

Haste removes the restriction that normally prevents a creature from attacking or using tap abilities the turn it comes under your control. This restriction is defined in CR 302.6, and Haste bypasses it as stated in CR 702.10a.

The key point is that Haste only matters for the turn the creature came under your control. If you attach an Equipment that grants Haste to a creature you already controlled from a previous turn, Haste is irrelevant — that creature could already attack. The relevant case is when the creature entered under your control this turn.

If a creature entered the battlefield under your control this turn and you then attach a Haste-granting Equipment to it (for example, by equipping it during your first main phase), it immediately has Haste and is free to attack when you declare attackers. The Equipment grants Haste as a continuous effect (CR 613.1), so the creature has Haste at the moment you check for attack eligibility.

Concrete example: You play Goblin Guide (no native Haste in this scenario — imagine a vanilla 2/2). You then activate the Equip ability of a Swiftfoot Boots to attach it to that 2/2. Since Swiftfoot Boots grants Haste, your 2/2 can now attack this turn despite having just entered the battlefield.

Note: The Equipment must actually be attached to the creature at the time attackers are declared for the Haste bonus to apply. If Haste is removed (e.g., the Equipment is unattached) before or during the declare attackers step, the creature would be illegal as an attacker if it arrived this turn (CR 508.1).