What is the motivation for Scala assignment evaluating to Unit rather than the value assigned?

scala functional-programming io assignment-operator
Graham Lea picture Graham Lea · Jan 4, 2010 · Viewed 7.6k times · Source

What is the motivation for Scala assignment evaluating to Unit rather than the value assigned?

A common pattern in I/O programming is to do things like this:

while ((bytesRead = in.read(buffer)) != -1) { ...

But this is not possible in Scala because...

bytesRead = in.read(buffer)

.. returns Unit, not the new value of bytesRead.

Seems like an interesting thing to leave out of a functional language. I am wondering why it was done so?

Answer

David Pollak picture David Pollak · Jan 4, 2010

I advocated for having assignments return the value assigned rather than unit. Martin and I went back and forth on it, but his argument was that putting a value on the stack just to pop it off 95% of the time was a waste of byte-codes and have a negative impact on performance.

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