Side-effect free containers for E

[Ralph Hartley]

> "Mark S. Miller" <markm@caplet.com>:
> OTOH, I'm scared of having both 
> 
>      container[key]= newValue
> 
> and
> 
>      container[key]:= newValue
>      
> be legal in the same language with very different meanings.

I'm afraid that would be right up there with = vs == in C on the bug
parade. If you add the first you must eliminate the second. I haven't
been able to think if a syntax that bakes the distinction clear but
still follows (historically based) intuition. Perhaps it is ok to make
it a little jarring to discourage thoughtless use, for example

[key]container := newvalue

On the other hand [key]= seems very natural. However it does require
that [key] be an operator:

container [key]=newvalue --> container := container [key] newvalue

so

container [key] newvalue --> container with(key,newValue)

So [key] acts as a binary operator to produce a new collection, as an
right unary operator to extract a value, and as a left unary operator
to produce an "L-value".

A little messy I admit.

Ralph Hartley