היה followed by ל followed by infinitive construct‏‏

[flockhat]

Hello,

It is extremely common for the infinitive construct to have the preposition ל before it. What does it mean when used in combination with היה immediately before it?

Like Genesis 15:12 ויהי השמש לבוא or Isaiah 2:5 ויהי השער לסגור בחשך.

 

[Drink]

It seems to mean "about to". The sun was about to set. The gate was about to be closed.