Let is used to temporarily hold a value in an EAL expression.

Let can be very useful, but be careful when using it and also when deleting objects or referencing a derived expression.

Cause

It's not a variable, it's a reference.

Also, look at derived associations that are used to create "shortcuts" in your model or to split complex navigations into parts.

Both Let references and combining these two can lead to unexpected results.

Example with Single Link Association

If you have a derived single link association called lastSubPart, derived like this:

self.subParts->last

then you have a method with this content:

let lp = self.lastSubPart in 
(
  self.subParts.add(newPart);
  newPart.Name = 'copy of ' + lp.Name
)

You might expect newPart.Name to hold 'copy of <name of the lastsubpart>', but it doesn't.

That's because lp is NOT holding the object - it's holding a reference to the subscription to the derivation to the object.

Solution

When the code does self.subParts.add(newPart), lp changes to point to the new object.

I.e. every time you use lp, the derived single link association is reevaluated.

let lp = self.lastSubPart->first in 
(
  self.subParts.add(newPart);
  newPart.Name = 'copy of ' + lp.Name
)

The first operator here converts the object to a set and then takes the first, which is the object itself, not the reference or subscription.