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[Caml-list] "in" question
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Date: -- (:)
From: Alessandro Baretta <alex@b...>
Subject: Re: [Caml-list] "in" question


mzukowski@yci.com wrote:
> I've been working through Jason Hickey's introduction to Objective Caml and
> don't quite get why the "in" is necessary in this construct:
> 
> # let sum i = let sum2 j = i + j in sum2 ;;
> val sum : int -> int -> int = <fun>

You definitely need "in"; here's why. The keyword "let" 
defines a binding. Now you will see the meaning of the above 
code if you indent it appropriately.
let sum i =     (* Bind "sum" with the following expr *)
   let sum2 j =   (* Wait a sec! First bind "sum2" with...*)
     i + j        (* ...this.  Now what about "sum"? *)
   in sum2        (* Alright. Now we bound "sum" as well   *)

> If I leave out the "in" I get a generic "Syntax error":
> 
> # let sum i = let sum2 j = i + j ;;
> Characters 31-33:
>   let sum i = let sum2 j = i + j ;;
>                                  ^^
> Syntax error

That's right. You declared you were going to bind "sum" with 
an expression, but you did not specify any expression. You 
just bound "i + j" with "sum2", not with "sum".

> I'm sure it's something fundamental about scoping I'm missing.  Please
> enlighten me.

I gave you an informal explanation. You can find an 
explanation of the formal semantics of the "let ... in ..." 
construct (alas, in french!) at 
"http://www.enseignement.polytechnique.fr/informatique/M2/lp/" 
(Cours 2).

Briefly stated, and all typing considerations set aside,
 > let x = <term1> in <term2>
is equivalent to
 > (fun x -> <term2>) <term1>

Let's apply this rule to the correct code.
 > # let sum i = let sum2 j = i + j in sum2
You first have to expand the definition of functions:
 > let sum = fun i -> let sum2 = fun j -> i + j in sum2
Then we can apply the above mentioned equivalence once...
 > let sum = fun i -> ((fun sum2 -> sum2) (fun j -> i + j))
                        ^^^^^^^^^^^^^^^^
...then we recognize an application of the identity function 
and obtain...
 > let sum = fun i -> fun j -> i + j
...which we can represent in a more compact fashio as...
 > let sum i j = i + j
...which is what we wanted.

Not really straightforward until you get the hang of 
functional languages. But it's cool, in the end.


> Thanks,
> 
> Monty

Have fun!

Alex

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