Pony has the ability to pattern match on classes and also assign results in match expressions (with let ... :), but is there a way to unpack values in match expressions? For example something like this?
actor Main
fun f(x: (Foo | Bar)): String =>
match x
| Foo(1) => "one"
| Foo(2) => "two"
| Foo(x) => x.string() // fails
else
"Bar"
end
The only alternative I can think of is a series of
actor Main
fun f(x: (Foo | Bar)): String =>
match x
| Foo(1) => "one"
| Foo(2) => "two"
| Bar => "Bar"
else
try
let f = x as Foo
f.number.string()
end
end
But that doesn't read as well, especially if there are multiple possible classes to match on.
I assume you have accompanying definitions like this:
class Foo is (Equatable[Foo box] & Stringable)
var value: I32 = 1
new create(value': I32) => value = value'
fun box eq(other: Foo box): Bool => value == other.value
fun string(): String iso^ => value.string()
primitive Bar
Then you can bind names for whole values of a certain type, like this:
actor Main
fun f(x: (Foo | Bar)): String =>
match x
| Foo(1) => "one"
| Foo(2) => "two"
| let x': Foo => x'.string()
else
"Bar"
end
I think this is not too bad in this particular case, but it's certainly not a true destructuring bind. Pony supports such patterns only for tuples, of the form (let first: First, let second: Second).
Related
I have this datatype
datatype json =
Num of real
| String of string
| False
| True
| Null
| Array of json list
| Object of (string * json) list
and this code
fun mymatch (jobj,str) =
case jobj of
Array [] => NONE
| Array(Object oh::[]) => mymatch (Object oh, str)
| Array (Object oh::ot) =>
if isSome (assoc(str, oh)) then assoc(str, oh) else mymatch (Array ot, str)
| Object xs => assoc (str, xs)
| _ => NONE
with this helper function
fun assoc (k, ls) =
case ls of
[] => NONE
| (a,b)::[] => if k = a then SOME b else NONE
| (a,b)::xs => if k = a then SOME b else assoc (k,xs)
which should take something like this
mymatch (Array [Object [("n", Num (4.0)),("b", True)],Object [("last", Num (4.0)),("foo", True)]],"foo")
and return a match on the string "foo", searching each Object in the Array. As you can see in the code, I'm really only handling the two things in json that qualify for a match, i.e., an Array that contains Objectss, then sending the Objects to be checked. This code works, but it's surely from the brutalist school of programming, i.e., it feels like a kludge. Why? Because of the case in mymatch where I need to recurse down through the Array
...
| Array (Object oh::ot) =>
if isSome (assoc(str, oh)) then assoc(str, oh) else mymatch (Array ot, str)
...
Until now, I've only dealt with recursion on lists where you check the car, then recurse on the cdr. Again, this code works, but I can sense I'm missing something. I need to check the head Object of Array and terminate if it matches; otherwise, keep recursing -- all within the option return world. Is there a more elegant way of doing this?
Write a function for matching the "innards" of arrays:
fun match_array (str, Object ob :: obs) = (case assoc (str, ob) of
NONE => match_array (str, obs)
| something => something)
| match_array _ = NONE;
then rewrite mymatch:
fun mymatch (str, Array a) = match_array (str, a)
| mymatch (str, Object ob) = assoc (str, ob)
| mymatch _ = NONE;
You can also simplify assoc a little bit:
fun assoc (k, []) = NONE
| assoc (k, (a,b)::xs) = if k = a then SOME b else assoc (k,xs);
I have two lists of equal length. I want to filter the elements of the first list by looking, if the element, with the same index in the second list, has a true boolean value.
Example:
[1,2,3,4,5]:int list
[true,false,false,true,false]:bool list
Expected result: [1,4]
I know two ways I could achieve this:
1) Write a function that takes two lists. For every element in the first list, that I want to append, check if the current(head) element of the second list is true.
2) Zip the two lists and filter it according to the boolean value.
There should be an easier to go about this, right?
Not really. The cleanest way to do this is probably
List.map (fn (x,y) => x) (List.filter (fn (x,y) => y) (ListPair.zip (L1,L2)))
or
List.map Option.valOf (List.filter Option.isSome (ListPair.map(fn (x,y) => if y then SOME x else NONE) (L1,L2)))
The recursive function isn't too bad, either:
fun foo ([],[]) = []
| foo ([],L) = raise Fail "Different lengths"
| foo (L,[]) = raise Fail "Different lengths"
| foo (x::xs, b::bs) = if b then x::foo(xs,bs) else foo(xs,bs)
Those are pretty much the two options you have; either recurse two lists at once, or combine them into one list of tuples and recurse that. There are several combinators you could use to achieve the latter.
val foo = [1,2,3,4,5];
val bar = [true,false,true,true,false];
val pairs = ListPair.zip (foo, bar)
Once zipped, here are two other ways you can do it:
val result = List.foldr (fn ((n,b), res) => if b then n::res else res) [] pairs
val result = List.mapPartial (fn (n,b) => if b then SOME n else NONE) pairs
The simplest is probably
ListPair.foldr (fn (x,y,z) => if y then x :: z else z) [] (L1, L2)
Don't know if ML has list comprehension, but if your language has it:
[ x | (x, True) <- zip xs ys ]
Ok, so I'm trying to change this function into Tail Recursive. The Definition I have of Tail Recursive is to use a "Local Helper Function" to accumulate my answer and return it without calling the primary function recursively.
these functions work properly.
fun same_string(s1 : string, s2 : string) =
s1 = s2
fun all_except_option (name, []) = NONE
| all_except_option (name, x::xs)=
case same_string (x , name) of
true => SOME xs
| false => case all_except_option(name,xs) of
NONE => NONE
| SOME z => SOME(x::z)
fun get_substitutions1 ([],name2) = [] (*get_substitutions2 is same but tail recursive *)
| get_substitutions1 (x::xs,name2) =
case all_except_option (name2,x) of
NONE => get_substitutions1 (xs,name2)
| SOME z => z # get_substitutions1(xs,name2)
So here are my attempts at tail recursion which do not work and I think I am missing something fairly basic that I am overlooking due to my lack of experience in SML.
fun get_substitutions2 (lst,name3) =
let fun aux (xs,acc) =
case all_except_option(name3,x::xs) of
NONE => aux(xs, acc)
| SOME z => aux(xs, z::acc)
in
aux(lst,[])
end
and
fun get_substitutions2 (lst,name3) =
let fun aux (xs,acc) =
case all_except_option(name3,x::xs) of
NONE => aux(xs, acc)
| SOME z => aux(xs, z#acc)
in
aux(lst,[""])
end
Both "get_substitutions" functions are supposed to do the same thing.
compare String1 to string list list, return single list made up of all lists containing String1 minus String1.
My attempts at using Tail Recursion have resulted in the following error.
Error: unbound variable or constructor: x
uncaught exception Error
raised at: ../compiler/TopLevel/interact/evalloop.sml:66.19-66.27
../compiler/TopLevel/interact/evalloop.sml:44.55
../compiler/TopLevel/interact/evalloop.sml:296.17-
Here are a few examples of calling get_substitutions2:
get_substitutions2 ([["foo"],["there"]], "foo"); (* = [] *)
get_substitutions2 ([["fred","fredrick","freddie","F","freddy"],["Will","William","Willy","Bill"]],"Bill"); (* = ["Will","William","Willy"] *)
get_substitutions2 ([["a","b"],["a","c"],["x","y"]], "a"); (* = ["c","b"] *)
You need to use the same patterns you had for get_substitutions1 in your aux function definition:
fun get_substitutions2 (lst,name3) =
let fun aux ([],acc) = acc (* BASE CASE *)
| aux (x::xs,acc) = (* BINDING x IN PATTERN *)
case all_except_option(name3,x) of
NONE => aux(xs, acc)
| SOME z => aux(xs, z#acc)
in
aux(lst,[])
end
I'm creating sum of list and using option in it. When I pass an empty list, I should get NONE or else SOME value.
I'm able to do that in the following way:
fun sum_list xs =
case xs of
[] => NONE
| x =>
let
fun slist x =
case x of
[] => 0
| x::xs' => x + slist xs'
in
SOME (slist x)
end
But I want to do it in the other way round using pattern matching, in which I want to eval the result of sum_list to see whether it is NONE or contains some other value.
I have tried in various ways but I cannot get a hang of how to do in that way.
I think what you currently have is very clear and easy to understand.
If you want to avoid using slist, you have to call sum_list recursively on the tail of the list, pattern-match on that option value and return appropriate results:
fun sum_list xs =
case xs of
[] => NONE
| x::xs' => (case (sum_list xs') of
NONE => SOME x
| SOME y => SOME (x+y))
I'm coding in SML for an assignment and I've done a few practice problems and I feel like I'm missing something- I feel like I'm using too many case statements. Here's what I'm doing and the problem statements for what I'm having trouble with.:
Write a function all_except_option, which takes a string and a string list. Return NONE if the string is not in the list, else return SOME lst where lst is like the argument list except the string is not in it.
fun all_except_option(str : string, lst : string list) =
case lst of
[] => NONE
| x::xs => case same_string(x, str) of
true => SOME xs
| false => case all_except_option(str, xs) of
NONE => NONE
| SOME y=> SOME (x::y)
Write a function get_substitutions1, which takes a string list list (a list of list of strings, the substitutions) and a string s and returns a string list. The result has all the strings that are in some list in substitutions that also has s, but s itself should not be in the result.
fun get_substitutions1(lst : string list list, s : string) =
case lst of
[] => []
| x::xs => case all_except_option(s, x) of
NONE => get_substitutions1(xs, s)
| SOME y => y # get_substitutions1(xs, s)
-
same_string is a provided function,
fun same_string(s1 : string, s2 : string) = s1 = s2
First of all I would start using pattern matching in the function definition
instead of having a "top-level" case statement. Its basically boils down to the
same thing after de-sugaring. Also I would get rid of the explicit type annotations, unless strictly needed:
fun all_except_option (str, []) = NONE
| all_except_option (str, x :: xs) =
case same_string(x, str) of
true => SOME xs
| false => case all_except_option(str, xs) of
NONE => NONE
| SOME y => SOME (x::y)
fun get_substitutions1 ([], s) = []
| get_substitutions1 (x :: xs, s) =
case all_except_option(s, x) of
NONE => get_substitutions1(xs, s)
| SOME y => y # get_substitutions1(xs, s)
If speed is not of importance, then you could merge the two cases in the first function:
fun all_except_option (str, []) = NONE
| all_except_option (str, x :: xs) =
case (same_string(x, str), all_except_option(str, xs)) of
(true, _) => SOME xs
| (false, NONE) => NONE
| (false, SOME y) => SOME (x::y)
But since you are using append (#), in the second function, and since it is not
tail recursive, I don't believe that it your major concern. Keep in mind that
append is potential "evil" and you should almost always use concatenation (and
then reverse your result when returning it) and tail recursion when possible (it
always is).
If you really like the explicit type annotations, then you could do it like this:
val rec all_except_option : string * string list -> string list option =
fn (str, []) => NONE
| (str, x :: xs) =>
case (same_string(x, str), all_except_option(str, xs)) of
(true, _) => SOME xs
| (false, NONE) => NONE
| (false, SOME y) => SOME (x::y)
val rec get_substitutions1 : string list list * string -> string list =
fn ([], s) => []
| (x :: xs, s) =>
case all_except_option(s, x) of
NONE => get_substitutions1(xs, s)
| SOME y => y # get_substitutions1(xs, s)
But that is just my preferred way, if I really have to add type annotations.
By the way, why on earth do you have the same_string function? You can just do the comparison directly instead. Using an auxilary function is just wierd, unless you plan to exchange it with some special logic at some point. However your function names doesn't sugest that.
In addition to what Jesper.Reenberg mentioned, I just wanted to mention that a match on a bool for true and false can be replaced with an if-then-else. However, some people consider if-then-else uglier than a case statement
fun same_string( s1: string, s2: string ) = if String.compare( s1, s2 ) = EQUAL then true else false
fun contains( [], s: string ) = false
| contains( h::t, s: string ) = if same_string( s, h ) then true else contains( t, s )
fun all_except_option_successfully( s: string, [] ) = []
| all_except_option_successfully( s: string, h::t ) = if same_string( s, h ) then t else ( h :: all_except_option_successfully( s, t ) )
fun all_except_option( s: string, [] ) = NONE
| all_except_option( s: string, h::t ) = if same_string( s, h ) then SOME t else if contains( t, s ) then SOME ( h :: all_except_option_successfully( s, t ) ) else NONE