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[Caml-list] "in" question
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 Date: 2002-11-22 (00:04) From: Alessandro Baretta Subject: Re: [Caml-list] "in" question
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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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