Re: pattern matching

Hi Ken,

What do you mean that "every language has an escape hatch to C"?  I'm not
familiar with YNot.  Judging by the context, I would assume that YNot is
some Ocaml module that allows Ocaml to directly implement C code.  I know
they do this occasionally in Python and Ruby for the sake of speed, but why
is this necessary in a language like Ocaml?  I thought Ocaml was just as
fast as anything written in C?

Ruby is a nice language, but at times it appears to be a little too
forgiving.  ( But please bear in mind that I am NOT a Ruby pro or expert,
so this is an amateur's opinion. )  So for example in most languages if you
have an array or list like this:

myList = [1, 2, 3, 4, 5] or [1; 2; 3; 4; 5] or you get the idea, and let's
say I do this: myList[10] or ( List.nth myList 10 ) or something to that
effect, I will get back an error!  Ruby has no
ArrayIndexOutOfBoundsException.  If I do that in Ruby, I'll just get nil.
 ( Ruby's version of null. )  I know from experience that C++, Java,
Python, and Ocaml will return an error message.  But Ruby permits this and
just returns nil.  If the user is prompted for integer input in Ruby, and
the user enters "Hello World" ( which certainly is not an integer! ) then
the string gets converted into the integer 0 if you call the *to_i*
method.  I found this to be rather annoying when I wrote this program where
I wanted to validate the user's input for correctness.  "Hello World" is
not valid input!  But Ruby just passed the string along into my integer
array, but the string got converted into 0, which is the default integer
value in Ruby.  So Ruby is very, very forgiving.  That's great if you're
new to programming.  But that forgiving nature can lead to some issues if
you want to validate user input, etc.  But again.... I'm no Ruby pro!  I'm
sure that a professional Rubyist would have some way to address this
particular issue.



On Tue, Mar 29, 2016 at 10:11 AM, Kenneth Miller [email protected]
[ocaml_beginners] <[email protected]> wrote:

>
>
> Patterns are expressions; you can write anything that fits the data type
> recursively, so long as you obey the type definition. To give you an idea,
> go back and look at Mr. Herr's definition; a head::second::tail is a list
> just as much as a [] is or a head::[], get it? Whatever it is that you
> pattern match, must deconstruct down to some legal construction component
> parts of a list in order that it can be said that the expression as a whole
> is a list.
>
>
> On Tuesday, March 29, 2016 11:06 AM, "Douglas Lewit [email protected]
> [ocaml_beginners]" <[email protected]> wrote:
>
>
>
> Some of it I got, and some I didn't.  But here's a question.  In pattern
> matching, can I simply write [head]?  Or am I obligated to write head :: [
> ].  They're interchangeable, right?  ( Although the syntax may require one
> rather than the other.  I would have to experiment with it to find out. )
>
> On Tue, Mar 29, 2016 at 4:02 AM, 'Mr. Herr' [email protected]
> [ocaml_beginners] <[email protected]> wrote:
>
>
> sorry, overlooked sub matching.
>
> You can match several elements in one pattern, just take care to have the
> "shorter" cases first.
>   | []  ->
>   | head :: [] -> (* equivalent to [head], all list patterns can be
> written with square brackets *)
>   | head :: second :: rest -> (* here rest can be [] *)
>
> This sounds like you did not understand the explanations I gave.
>
> /Str.
>
> On 29.03.2016 10:18, 'Mr. Herr' [email protected] [ocaml_beginners]
> wrote:
>
>
>
>
> On 29.03.2016 06:41, Douglas Lewit [email protected] [ocaml_beginners]
> wrote:
>
>
> I also gave something like this a try, but I ended up with a couple of
> error messages:
>
> *let pack list = match list with*
> *|[ ] -> [ ]*
> *|head :: tail -> match tail with*
> *|[ ] -> [ ]*
> *headOfTail :: tailOfTail -> if head = headOfTail then..... ( more code )*
>
> I thought it was clever, but either I just ended up with an empty list, a
> one-element list, or the compiler just rejected it.  I'm starting to feel a
> little dumb!  But I'll keep at it.
>
> tailOfTail is a list, not an element!
>
> /Str.
>
>
>
>
>
> 
>