Learn French: Il a installé un hamac histoire de dormir dehors. - He installed a hammock just to sleep outside.

While pour (to / in order to) is perfectly correct, histoire de adds a casual, lighthearted nuance. It translates closely to "just to" or "for the sake of," making his choice to sleep outside sound like a relaxed whim or a simple idea rather than a serious goal.

Is histoire de appropriate for all contexts, such as formal writing?

No, it is a colloquial expression. You will hear it constantly in everyday spoken French, but in a formal essay or a professional setting, you should stick to pour or the slightly more elevated afin de.

Why is dormir left in the infinitive here?

Because it immediately follows the preposition de. In French, when a verb follows a preposition like de, à, or pour, it must remain in its infinitive (unconjugated) form. This structure works perfectly here because the person installing the hammock and the person sleeping outside are the exact same subject.

When speaking this sentence aloud, do I make a liaison between un and hamac?

No, you do not. The word hamac begins with an "h aspiré" (aspirated H). Even though the letter "h" is silent in French, an aspirated H acts like a consonant wall that prevents liaisons. You must pronounce the words separately as un followed by hamac, rather than blending them together into "un-namac".