Re: [Felix-language] [felix] New loop

[john s. <sk...@us...>]

On 22/08/2014, at 11:35 PM, srean wrote:

> Looks good, all of it. What is this BTW
> 
> let ?v = x in 0 <= v and v < 10
> 
> could you break that up into its parts.


Sure: its a let expression:

	let pattern = value in expression

For example:

	let ?x = 1 in x + x

has the value 2.

Let expression has a very low precedence, the lowest in fact.
Which means the RHS expression "goes on forever" :)
Until ended by end of statement or whatever.

It is syntactic sugar for:

	match value with
	| pattern => expression
	endmatch

which is why you need to write let ?x with a question mark,
to denote a pattern variable.

Hence you can also write:

	let ?x, _ = 1,2 in x + x

since it just means

	match 1,2 with
	| ?x, _ => x + x
	endmatch


And of course

	cond1 and cond2

means what it says:  the logical conjunction of two predicates,
i.e. bool expressions.

you can also do this:

	let Some ?x = optional in x

which will throw an exception if the optional bit happens
to be None, just as a failed match throws an exception.

let .. in .. is the standard binder used in ML like functional
languages. Ocaml and Scheme and Felix all have it.

Haskell uses a backwards let, Haskell being backwards ...

	x + x where x = 1

which is the same meaning as 

	let ?x = 1 in x + x


let bindings crap all over block structure, because the temporary
variable goes out of scope after the expression. So you can write
nice stuff like

	let ?x = 
		let ?y = 3 in y + y
	in
		x + x

which of course has value 12 since y is 3, x is 6 and the whole
thing is worth 12. With block structure:

	var y = 3;
	var x = y + y;
	var result = x + x;

but the problem is y and x remain in scope. In Felix you can hide them:

	var result = 
		#{ var x = #{var y = 1; y + y }; x + x }
	;

but the let form is easier :)

[Note that #f means the same as f(), saves many hassles as above.
A { ... } form ending in an expression is a function which returns the
final expression. If it ends in a statement its a procedure, unless
it ends in a return statement returning a value in which case its
a function .. :]


--
john skaller
sk...@us...
http://felix-lang.org