I have a dictionary where the key is the name of the field, and the value is this field value. Is it possible to somehow explicitly indicate to which field what value to assign using this dictionary? Or is the only way to stick to the sequence of field declarations when passing the second argument to the FSharpValue.MakeUnion?
Two different approaches:
If the names match exactly, and you're willing to "stick to the sequence of field declarations when passing the second argument to the FSharpValue.MakeUnion", you can do this:
open FSharp.Reflection
let instantiate (unionCase : UnionCaseInfo) (dict : Map<_, _>) =
let args =
unionCase.GetFields()
|> Array.map (fun field ->
dict.[field.Name] :> obj)
FSharpValue.MakeUnion(unionCase, args)
Example usage:
type MyType = UnionCase of a : int * b : int * c : int
let unionCase =
FSharpType.GetUnionCases(typeof<MyType>)
|> Array.exactlyOne
let dict =
Map [
"a", 1
"b", 2
"c", 3
]
instantiate unionCase dict
:?> MyType
|> printfn "%A"
Output is:
UnionCase (1, 2, 3)
If you want to avoid reflection, you can assign to the fields in any order you want, like this:
UnionCase (
c = dict.["c"],
b = dict.["b"],
a = dict.["a"])
Related
In OCaml I can define the following type:
type mytype = Base of int
| Branch of (int * (collection -> collection))
and collection = mytype list
Assuming I define a comparison function based on the int value of each constructor, how can I transform collection to be a Set instead of a list?
This is one of the cases where you will need to use recursive modules. In fact you can see that this is the actual example you get in the documentation of the feature. So something along these lines should do it:
module rec Mytype : sig
type t = Base ...
val compare : t -> t -> int
end = struct
type t = Base ...
let compare v0 v1 = ...
end
and Collection : Set.S with type elt = Mytype.t
= Set.Make (Mytype)
Code:
type t_struct struct {
player string
id int
}
func main() {
dataA := make(map[string]t_struct)
dataB := make(map[string]*t_struct)
var playerA t_struct
playerA.player = "tom"
playerA.id = 1
dataA["classA"] = playerA
dataA["classA"].id = 2 // ERROR, why?
playerB := new(t_struct)
dataB["classB"] = playerB
dataB["classB"].player = "rick"
dataB["classB"].id = 3
}
And got error:
cannot assign to dataA["classA"].id
I wonder why dataA["classA"].id = 2 not worked but dataB["classB"].id = 3 did? Is it the only way to save struct pointer into map if you want to modify member value of it?
The expression dataA["classA"] if of type t_struct. That means that
dataA["classA"].id = 2
is equvalent to e.g.
t_struct{"some player", 42}.id = 2
Playground
IOW, the struct value has no "home" and changing its field cannot be persisted. As the only possibility is a programmer's mistake, the compiler flags an error.
OTOH:
dataB["classB"]
has type *t_struct. Then
dataB["classB"].id = 3
is equivalent to
(*t_struct)(somePointer).id = 3
I.e., this lvalue has a "home". It's where the pointer points to. The change to the field will be "recorded" there and it is thus a valid Go operation (simple assignment to a struct field where the struct is referenced by a pointer).
I need to use hashtable of mutable variable in Ocaml, but it doesn't work out.
let link = Hashtbl.create 3;;
let a = ref [1;2];;
let b = ref [3;4];;
Hashtbl.add link a b;;
# Hashtbl.mem link a;;
- : bool = true
# a := 5::!a;;
- : unit = ()
# Hashtbl.mem link a;;
- : bool = false
Is there any way to make it works?
You can use the functorial interface, which lets you supply your own hash and equality functions. Then you write functions that are based only on the non-mutable parts of your values. In this example, there are no non-mutable parts. So, it's not especially clear what you're expecting to find in the table. But in a more realistic example (in my experience) there are non-mutable parts that you can use.
If there aren't any non-mutable parts, you can add them specifically for use in hashing. You could add an arbitrary unique integer to each value, for example.
It's also possible to do hashing based on physical equality (==), which has a reasonable definition for references (and other mutable values). You have to be careful with it, though, as physical equality is a little tricky. For example, you can't use the physical address of a value as your hash key--an address can change at any time due to garbage collection.
Mutable variables that may happen to have the same content can still be distinguished because they are stored at different locations in memory. They can be compared with the physical equality operator (==). However, OCaml doesn't provide anything better than equality, it doesn't provide a nontrivial hash function or order on references, so the only data structure you can build to store references is an association list of some form, with $\Theta(n)$ access time for most uses.
(You can actually get at the underlying pointer if you play dirty. But the pointer can move under your feet. There is a way to make use of it nonetheless, but if you need to ask, you shouldn't use it. And you aren't desperate enough for that anyway.)
It would be easy to compare references if two distinct references had a distinct content. So make it so! Add a unique identifier to your references. Keep a global counter, increment it by 1 each time you create a reference, and store the counter value with the data. Now your references can be indexed by their counter value.
let counter = ref 0
let new_var x = incr counter; ref (!counter, x)
let var_value v = snd !v
let update_var v x = v := (fst !v, x)
let hash_var v = Hashtbl.hash (fst !v)
For better type safety and improved efficiency, define a data structure containing a counter value and an item.
let counter = ref 0
type counter = int
type 'a variable = {
key : counter;
mutable data : 'a;
}
let new_var x = incr counter; {key = !counter; data = x}
let hash_var v = Hashtbl.hash v.key
You can put the code above in a module and make the counter type abstract. Also, you can define a hash table module using the Hashtbl functorial interface. Here's another way to define variables and a hash table structure on them with a cleaner, more modular structure.
module Counter = (struct
type t = int
let counter = ref 0
let next () = incr counter; !counter
let value c = c
end : sig
type t
val next : unit -> t
val value : t -> int
end)
module Variable = struct
type 'a variable = {
mutable data : 'a;
key : Counter.t;
}
let make x = {key = Counter.next(); data = x}
let update v x = v.data <- x
let get v = v.data
let equal v1 v2 = v1 == v2
let hash v = Counter.value v.key
let compare v1 v2 = Counter.value v2.key - Counter.value v1.key
end
module Make = functor(A : sig type t end) -> struct
module M = struct
type t = A.t Variable.variable
include Variable
end
module Hashtbl = Hashtbl.Make(M)
module Set = Set.Make(M)
module Map = Map.Make(M)
end
We need the intermediate module Variable because the type variable is parametric and the standard library data structure functors (Hashtbl.Make, Set.Make, Map.Make) are only defined for type constructors with no argument. Here's an interface that exposes both the polymorphic interface (with the associated functions, but no data structures) and a functor to build any number of monomorphic instances, with an associated hash table (and set, and map) type.
module Variable : sig
type 'a variable
val make : 'a -> 'a variable
val update : 'a variable -> 'a -> unit
val get : 'a variable -> 'a
val equal : 'a -> 'a -> bool
val hash : 'a variable -> int
val compare : 'a variable -> 'b variable -> int
end
module Make : functor(A : sig type t end) -> sig
module M : sig
type t = A.t variable.variable
val make : A.t -> t
val update : t -> A.t -> unit
val get : t -> A.t
val equal : t -> t -> bool
val hash : t -> int
val compare : t -> t -> int
end
module Hashtbl : Hashtbl.S with type key = M.t
module Set : Set.S with type key = M.t
module Map : Map.S with type key = M.t
end
Note that if you expect that your program may generate more than 2^30 variables during a run, an int won't cut it. You need two int values to make a 2^60-bit value, or an Int64.t.
Note that if your program is multithreaded, you need a lock around the counter, to make the incrementation and lookup atomic.
I'm writing an F# dsl for SQL (http://github.com/kolosy/furious).
A select statement would look like this:
type person = {
personId: string
firstname: string
lastname: string
homeAddress: address
workAddress: address
altAddresses: address seq
}
and address = {
addressId: string
street1: string
zip: string
}
let (neighbor: person seq) =
db.Yield <# Seq.filter (fun p -> p.homeAddress.zip = '60614') #>
The obvious (and silly) question is... How do I parametrize the quotation?
If I just somehting like:
let z = "60614"
let (neighbor: person seq) =
db.Yield <# Seq.filter (fun p -> p.homeAddress.zip = z) #>
then z gets resolved into a static property accessor (PropertyGet(None, String z, [])). I need something that will let me retrieve the value of the variable/let binding based solely on the quotation. Ideas?
Quotations are not my forte, but check out the difference here:
let z = "60614"
let foo = <# List.filter (fun s -> s = z) #>
printfn "%A" foo
let foo2 =
let z = z
<# List.filter (fun s -> s = z) #>
printfn "%A" foo2
I think maybe having 'z' be local to the expression means the value is captured, rather than a property reference.
In addition to what Brian wrote - I believe that the encoding of access to global let bound values is also pretty stable and they will quite likely continue to be encoded as PropGet in the future.
This means that you could support this case explicitly in your translator and add a simple pre-processing step to get values of these properties. This can be done using ExprShape (which allows you to fully traverse quotation just using 4 cases). This would allow your DSL to support the general case as well.
The following function traverses quotation and replaces access to global lets with their value:
open Microsoft.FSharp.Quotations
let rec expand e =
match e with
// Extract value of global 'let' bound symbols
| Patterns.PropertyGet(None, pi, []) ->
Expr.Value(pi.GetValue(null, [| |]), e.Type)
// standard recursive processing of quotations
| ExprShape.ShapeCombination(a, b) ->
ExprShape.RebuildShapeCombination(a, b |> List.map expand)
| ExprShape.ShapeLambda(v, b) -> Expr.Lambda(v, expand b)
| ExprShape.ShapeVar(v) -> Expr.Var(v)
Then you can write the following to get a quotation that contains value instead of PropGet:
let z = 5
let eOrig = <# Seq.filter (fun p -> p = z) [ 1 .. 10 ]#>
let eNice = expand eOrig
I am having a bit of a problem with a functor (and it's resultant type). Below, I have a Set functor that uses an Ordered type. I actually used the set.ml that comes with OCaml for some guidance, but I seem to be doing everything ahem right. I created an Ordered module with integers and applied it to the Set functor to get the last module on this code sample, IntSet.
The next line fails, when I try to insert an integer. I get the following type error:
Error: This expression has type int but is here used with type
SetInt.elt = Set(OrdInt).elt
Don't get me wrong, the type system is correct here. The top level reports that the type of the SetInt.elt is Set(OrdInt).elt, but when I do the same operations to set up a Set using the one provided by OCaml the 'same' line is, SetInt.elt = OrderedInt.t. Seems like I should be getting SetInt.elt = Ordered.t.
This is so simple, I'm probably just missing some stupid detail! argh!
Please Note: I have simplified the member/insert functions here since this issue has to do with types.
module type Ordered =
sig
type t
val lt : t -> t -> bool
val eq : t -> t -> bool
val leq : t -> t -> bool
end
module type S =
sig
type elt
type t
exception Already_Exists
val empty : t
val insert : elt -> t -> t
val member : elt -> t -> bool
end
module Set (Elt:Ordered) : S =
struct
type elt = Elt.t
type t = Leaf | Node of t * elt * t
exception Already_Exists
let empty = Leaf
let insert e t = t
let member e t = false
end
module OrdInt : Ordered =
struct
type t = int
let lt a b = a < b
let eq a b = a = b
let leq a b = a <= b
end
module IntSet = Set (OrdInt)
(* line that fails *)
let one_elm = IntSet.insert 1 IntSet.empty
You need to change these two lines
module Set (Elt:Ordered) : S =
module OrdInt : Ordered =
to
module Set (Elt:Ordered) : S with type elt = Elt.t =
module OrdInt : Ordered with type t = int =
Without these, the modules will not have signatures that expose the types elt and t as int.
[Edit]:
The set.ml doesn't have the 'with' bit, because there's a sml.mli, which declares the signature for the functor and it does have the 'with'. Also, OrdInt doesn't need 'with' if you don't explicitly specify a signature for it, like this:
module OrdInt =
You can also construct the set by defining the module in place:
module IntSet = Set (struct
type t = int
let lt a b = a < b
let eq a b = a = b
let leq a b = a <= b
end)