Re: Expression-local variables and 'let'

Quoting Daniel Bonniot <[email protected]>:

> val balance = currentBalance() in display(balance, warn: balance < 0);
> display(val balance = currentBalance(), warn: balance < 0);
> 
> Yes, I think the second one is more readable. The first thing you see is 
> display, so you know straight away what we are trying to do, while in 
> the other version it's burried deep down.

I find the first one to be more readable than the second. I guess we can just
"agree to disagree." But, I will play along with you below.

> Let's call balance the current balance. Now, display that balance, and 
> warn the user if that balance is negative.
> 
> vs
> 
> Display the balance which is the current balance, and warn the user if 
> that balance is negative.
> 
> The first version is more complex: it starts by posing a binding, and 
> then goes on with the main part. I think that's akin to first proving a 
> lemma when what you want is to prove a certain theorem. If the lemma is 

I found I could understand the first one faster than the second one. Also,
according to Flesch-Kincaid readability test as implemented in MS Word, the
first one is easier to read (grade 5.5) than the second one (grade 6.6).

> With a less serious example: I would say "There is this person who is 
> the brother in law of the boss of my wife's cousin. Well, i'm going to 
> play music with him at his place", but I wouldn't say "There is this 
> person who is my father. Well, i'm going to play music with him at his 
> place", but "I'm going to play music with my father at his place" (in a 
> program, you would write "at my father's place", because there is no 
> implicit binder like it/him/her, unless you program in perl ;-)

You are mixing things around. Your first sentence is like:
    let person = father() in
    play_music(person, home(person));
Your second sentence is like:
    play_music(father(), home(father());

It seems the second one should be "I'm going to play music with this person who
is my father at that person's place":

    play_music(val person = father(), home(person));
    
> >You have to have seperate grammar constructs "single variable declaration"
> (for parameters) and "variable declaration" (for the general case). 
> >
> That's a minor annoyance, especially given that their semantics 
> scope-wise is different.

I mean, it makes it harder for someone learning the language to understand the
syntax. 

> >Also, you can't
> >say that this is a generalization of the syntax for for(...) because you
> can declare multiple variables at once in a for loop:
> >
> >    for (int x = 1, y = 0;;);
> >  
> It's a proper generalization of 'using', still. It also generalizes 
> 'while' if I'm not mistaken, since you can then also implement it as a 
> user-defined function.

I don't think you can implement 'while' as a user-defined function with the same
syntax since 'while' needs to be able to evaluate the expression in the
parentheses multiple times. You can do 'if (x) { ... }' (no else) though.

> >>If you need to define several variables in a limited scope, you can 
> >>already use the C-syntax idiom (should I say Algol?):
> >>
> >>{
> >>   let x = ...;
> >>   let y = ...;
> >>   ...
> >>} // scope ends for x and y
> >>    
> >>
> >
> >I think everybody hates that syntax, which is why nobody ever uses it.
> >
> Yes, it's rarely used, but I'm not sure it's because the syntax is ugly. 
> It can also be because methods should be short anyway, so you probably 
> write a helper method instead of creating a nested scope. And for what's 
> left, the extra trouble is probably not worth the extra scoping you get 
> (if a method is 3-5 lines, then usage is easily spotted).

This is an argument against the construct in general, not the "let..in" syntax
for it.

> >Main Entry   : let..in
> >Pronunciation: 'let 'in
> >Etymology    : ML
> >Definition   : magic solution ;)
> >  
> >
> I'm not convinced yet, unfortunately.

I know, I hope you saw that this was just a joke.

- Brian




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