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[Caml-list] semi-colons and begin
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Date: -- (:)
From: Gerd Stolpmann <gerd@g...>
Subject: Re: [Caml-list] semi-colons and begin
On Wed, 04 Apr 2001, Chris Hecker wrote:
>I must admit I'm still slightly confused about the semicolon thing, even after writing a fair amount of caml code.  The current sticking point is that 
>
># if false then print_int 2; print_int 3;;
>3- : unit = ()
>
># if false then print_int 2; print_int 3 else ();;
>Characters 39-43:
>Syntax error
># 
>
># if false then begin print_int 2; print_int 3 end;;
>- : unit = ()
># if false then begin print_int 2; print_int 3 end else ();;
>- : unit = ()
>
>The "BNF" grammar implies the first and second tests should compile and print nothing,
>since expr = expr ; expr, although Xavier et al. have said the BNF in the docs
>isn't really accurate. 

The BNF is ambigous (as most grammars). So you can't say which is the right way
to set the parentheses:

(if false then print_int 2); print_int 3   -- OR
if false then (print_int 2; print_int 3)

Both ways are correct if you only look at the grammar. However, in the O'Caml
manual there is a precedence table (section 6.7) determining that "if" has
higher precedence than ";". So the first way is what the compiler interprets.

>
>also thought begin...end was needed in a
>pattern-match, but apparently expr ; expr works there? 

In the precedence table "match" has lower precedence than ";".

>Is there a chart or something that says exactly where they're allowed?  I'm
>afraid of introducing a subtle bug in my program.  In C I always use brackets
>with my if statements to avoid the similar problem, so I suppose I could use
>begin..end everywhere, but yuck! 

You can use begin ... end (or (...) which is the same) everywhere in
expressions. But I don't recommend it because it makes code unreadable which is
another cause for errors.

I personally have no problems with "if" because it is in the same style as in
the first structured programming language I've learned (Pascal).

If you have problems I would recommend to use Emacs as editor with one of the
two O'Caml modes. It automatically indents the code, so you see:

if false then print_int 2;
print_int 3

and _not_

if false then print_int 2;
  print_int 3

which would mean that you did it wrong.

Gerd
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Gerd Stolpmann      Telefon: +49 6151 997705 (privat)
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