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Date: -- (:)
From: Pierre Weis <weis@p...>
Subject: Re: Constructive criticism
Thank you very much for the bugs report, we'll correct them as soon as
possible.

It's a pleasure to answer to your numerous and so constructive suggestions:

Patterns in fun matches:
------------------------
>        - The following error seems difficult to understand:
>
>                #let testit = fun | 4 -> 2 | 1 | 2 -> 3;;
>                Toplevel input:
>                >let testit = fun | 4 -> 2 | 1 | 2 -> 3;;
>                >                              ^
>                Syntax error.
>                #
          
>          Why is this OK with "function" but not with "fun"? It is
>          not possible to match something to the symbol "|" is it?      

The rule of thumb is that complex patterns have to be bracketed when
used inside ``fun'' pattern matchings. And since ``or'' patterns, that
is patterns separated by the or symbol ``|'', are not considered basic.

Technically, this strange treatment of patterns is mainly due to Yacc
restrictions, and partly due to real syntactic ambiguities, since fun
patterns may bind several parameters (when function patterns only bind
one). Consider the pattern ``C D'', where C and D are constructors.
This patterns is syntactically ambiguous: is it the application of
constructor C to constructor D, or two successive parameters,
parameter C followed by parameter D ? In a function pattern the first
interpretation is used, in a fun pattern the second one is assumed
(hence, when you need the first meaning in a fun pattern you must
bracket the application of constructor C to D, writing (C D)).

Library considerations:
-----------------------
>        - The standard library seems to miss some routines that
>          I would expect / like to see through symmetry

Yes, the Caml Light library has been designed to be as light as possible!
Some one-liner functions have been omitted (such as your copy_string,
which is just (function s -> s ^ "")). Some other useful functions
may not be provided in the standard library (for instance sprintf is
only available via the contrib/libstr library, as the function str__format).

In contrast, the Caml V3.1 library has more functions. Almost all the
functions you suggest to add to the standard libray have a counterpart
in this library, some time a  bit more general than yours. I give the
Caml V3.1 version when it is better than yours:

> let list_of_n n x = list_of_vect (make_vect n x);;

Your list_of_n function is not optimal, since you allocate a vector
that you immediately throw away to get the list of its elements. It it
more efficent to create the list directly.

Anyway, list_of_n is usually named replicate in Caml:

let rec replicate n x =
 if n <= 0 then [] else x :: replicate (n - 1) x;;

>              let map_list_count f l =
>                let rec work n = function
>                   | [] -> []
>                   | h::t -> (f n h)::(work (n+1) t)
>                in work 0 l;;

Your map_list_count function is a special case of the more general
map_i functional defined as:

let map_i f =
 let rec map_i_rec i = function
     [] -> [] | x::l -> f i x::map_i_rec (i+1) l in
 map_i_rec;;
map_i : (int -> 'a -> 'b) -> int -> 'a list -> 'b list = <fun>

The corresponding primitive for vectors is map_i_vect_list to return a list:

let map_i_vect_list f i0 v =
 let rec map_i_rec n =
  if n >= vect_length v then [] else f n v.(n)::map_i_rec (succ n) in
 map_i_rec i0;;
map_i_vect_list : (int -> 'a -> 'b) -> int -> 'a vect -> 'b list = <fun>

or map_i_vect to return a new vector:

let map_i_vect f i0 v =
  let len = vect_length v - i0 in
  if len <= 0 then [| |] else
  let result = make_vect len (f i0 v.(i0)) in
  for i = i0 + 1 to i0 + len - 1 do result.(i - i0) <- f i v.(i) done;
  result;;
map_i_vect : (int -> 'a -> 'b) -> int -> 'a vect -> 'b vect = <fun>

>              let rec list_do f = function
Your list_do that applies a function to the ellements of a list in
reverse order is usually called rev_do_list:
              let rec rev_do_list f = function
                | [] -> ()
                | h::t -> rev_do_list f t; f h; ();;

extract_list is known as filter in Caml V3.1.

create_numlist corresponds to interval:
(* Lists of consecutive integers *)
let interval n m =
 let rec interval_n (l,m) =
  if n > m then l else interval_n (m::l,pred m) in
 interval_n ([],m);;

let range = interval 1;;

Packed arrays:
--------------

>Probably futile wish list for implementors with infinite amounts of time.

>        - Packed arrays of enumerated (sum) types with only a small number of 
>          possibilities and with no arguements -- like boolean.

Why do you want them packed ? You can use vectors. I agree that in
some cases vectors waste memory locations, in particular for caracters
(but Caml provides strings).
In some cases, you can consider to use bit_vectors (I can send you such a
library if you want), otherwise you can use integers and logical
shift operations as in C, but this is very low-level. The best
solution is probably to consider these packed arrays as a compiler
optimisation, but is it worth the work it needs ?

Unary minus:
------------
>        - I would like to be able to not have to bracket the unary minus
>          before a constant in a function application.

>                eg. let m1 = num_of_int -1;;

Well, I presume that you want to use the same symbol for unary and
binary minus (that is the mathematical ``-''). In this case you may be
able to distinguish between those two. If you denote function
application by mere juxtaposition, then f - 1 can either be the
application of function f to the number -1 or the subtraction of 1 to
the integer variable f. You may want to distinguish with spaces: then
-1 is a single token and f -1 is an application, while f - 1 is a
binary operation. The problem is that now f-1 is an application, and
thus x*y-1 is understood as x * (y (-1)). This is strange. An
admissible solution could be to give the same treatment to other
binary operators...

Extended pattern matching:
--------------------------

>        - I would like to be able to use a symbol twice in 
>          pattern matching. For instance, 

>                let compare = fun
>                        | x x -> 0
>                        | x y -> if x>y then 1 else -1
>                ;;

You can encode this using a guard (a ``when'' side-condition) in your
pattern matching clause:
let compare = function
  | (x, y) when x = y -> 0
  | (x, y) -> if x > y then 1 else -1;;
compare : 'a * 'a -> int = <fun>

>         I realise that this involves (in some cases) a generalised
>          comparison which is not always possible for functions, but a 
>          generalised "=" is used elsewhere, and the exception for functional
>          values seems like a reasonable solution.

You're right. It is conceivable to add this kind of ``non-linear''
patterns to Caml: the compiler just has to generate the right ``when''
conditions. This may be done some day. This was not done to maintain a
good property of Caml pattern matching: pattern matching selection
is fast, and in particular it cannot loop for ever. In the presence of
cyclic data, the equality test generated by non linear patterns may
need an unpredictable amount of computation to complete, or may even
loop for ever.

In fact we prefered to implement guards rather non linear patterns,
since guards are more general, allowing to test not only variable
equalities but arbitrary boolean conditions (in particular they allow
user defined equality rather than the built-in ad hoc equality).

Continue in patterns:
---------------------

>        - I would like someway of handling the following nicely:

>                | a b when let (success,res) = big_fn a b in success
>                        ->  let (success,res) = big_fn a b in res

>          Perhaps something like

>                | a b -> let (success,res) = big_fn a b in
>                        if success then res else fail

>          where "fail" would pass one on to the next condition to be tested.

This is known as the ``continue in pattern matching'' feature in the
Caml community. It has been implemented in Caml V3.1 some time ago.
You simply use ``continue'' instead of your ``fail'', and the semantic
is exactly what you wanted.

In fact, a generalised version is implemented: continue statements may
refer to a specific pattern matching, which is useful in case of
nested pattern matching.

Thus your example is written as:

let big_fn a b = if a then (a, b + 1) else (false, b);;
Value big_fn is <fun> : 'a * int -> 'a * int

let g = function continuing g
 | (a,b) -> let (success,res) = big_fn (a, b) in
    if success then res else continue g
 | _ -> 2;;
Value g is <fun> : bool * int -> int

#g (true, 4);;
5 : int

#g (false, 4);;
2 : int

In some cases, you can simulate the continue statement with some
rewritting of your function and a ``try raise'' construct. Something like:

let g x = try continuing_g x with continue -> rest_of_patterns x

let continuing_g = function
 | (a,b) -> let (success,res) = big_fn (a, b) in
    if success then res else raise continue
 | z -> rest_of_patterns z

and rest_of_patterns _ = 2

This trick is not a solution: it is untractable for complex pattern matching.

Generalized orpats:
-------------------
>        - I would like to be able to write functions that are in some
>          way symmetric between parameters. For instance, instead of writing

>                fun
>                        | Null x -> x
>                        | x Null -> x
>                        | x y -> messy_implementation_of_add x y

>          I would like to be able to merge the two first lines into one line.
>          I can't suggest a nice syntax for this that makes sense for more
>          than two parameters and which is still intuitive. This problem
>          is made less of a problem if the next point is done: 

Your problem is just to generalize ``or'' patterns, to allow them to
bind variables. Once more this is possible in Caml V3.1.

Given:

type nat = Null | Succ of nat;;

let messy_implementation_of_add x y = Null;;

You may use ``or'' patterns that bind variables to write:

let f = function
  | Null, x | x, Null -> x
  | x, y -> messy_implementation_of_add x y;;
Value f is <fun> : nat * nat -> nat

#f (Null, Succ Null);;
(Succ Null) : nat

#f (Succ Null, Null);;
(Succ Null) : nat

#f (Succ Null, Succ Null);;
Null : nat

Definitions of record values:
-----------------------------

>        - I would like to be able to define a new structure instance with
>          a declaration like { x ; Field3=5 ; Field7 = 9 } which would make a
>          copy of the structure x, whilst changing the two given fields to the
>          appropriate values. Why? So I can write a program that has lots of
>          things that it will eventually do with slight changes to a structure,
>          and I don't have to go through and change routines that are 
>          irrelevant to the change.

This a well known problem: allocation of record values is much more
messy than allocation of sum type values. As you mentioned, this is
particularly evident when you have to modify a few part of a given
record.

However, in Caml you get the capability to treat the fields that have to be
modified in an imperative way: you just declare those fields ``mutable''
when defining the record type. This way you share the record and its
modified version, and just have to change what have to be modified.
Your example would be something like:
r -> r.Field3 <- 5; r.Field7 <- 9; r

Unfortunately, when you need a new (fresh) data structure, this
solution does not work.
Some partial solutions have been proposed: for instance Caml V3.1 provides a
short-hand to define the content of a record field: when no expression
is associated to a field name then a variable expression equal to the
field name is assumed.
This way { x; y} is read as {x = x; y = y}. Your example is then a bit
simplified 
{ Field1; Field2; Field3; Field4; Field5; Field6; Field7 } ->
{ Field1; Field2; Field3 = 5 ; Field4; Field5; Field6; Field7 = 9 }

Unfortunately, this does not solve your maintenance problem.
By contrast, your generalised ellipse is an exiting alternative, since
it allows an easy modification of allocating routines.

Overloading:
------------
>Really grasping at straws:

>        - I would like to be able to perform "function overloading" as
>          in C++. Furthurmore, greedy person that I am, I would like this
>          overloaded-ability to automagically be passed through into 
>          functions that use the overloaded function ...

This is really difficult to provide, and is a long-time research problem.
However, there exists a restricted form of overloading in Caml V3.1
that we called ``static overloading''.

We recently made progress on that point and proposed new type systems
and new compilation schemes to provide a general overloading facility
(see the article ``Extensional Polymorphism'' in POPL 95).
This work still has to be fully incorporated into a working Caml system.

>Maybe I should go to caml rather than
>          caml-light.
Use both!

Pierre Weis
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