I guess it's now Object.ToString() but I'm unsure... Thanks!
I think the only way to get any_to_string functionality is with the "%A" formatter. The warning tells you this.
let any_to_string = sprintf "%A"
This does not invoke .ToString(). For 'simple' types like lists the .ToString() already yields a good representation. But when using your own custom types, the %A formatter is much more useful. For instance, in case of tree structures it walks along the tree.
If you do want to invoke .ToString() on an object, you can use the '%O' formatter.
Example:
type Tree = Node of Tree * Tree | Leaf
let myTree = Node(Node(Leaf,Leaf),Node(Leaf,Node(Leaf,Leaf)))
and in FSI:
> myTree.ToString();;
val it : string = "FSI_0002+Tree+Node"
> sprintf "%O" myTree;;
val it : string = "FSI_0002+Tree+Node"
> sprintf "%A" myTree;;
val it : string = "Node (Node (Leaf,Leaf),Node (Leaf,Node (Leaf,Leaf)))"
you can use sprintf:
let a = [1;2;3]
let b = sprintf "%A" a
What about the string function?
> string [1..3];;
val it : string = "[1; 2; 3]"
Related
I'm trying to go from:
sprintf "%3.1f" myNumber
to:
sprintf myFormatter myNumber
which is not possible
I have a situation where number precision depends on some settings, so I would like to be able to create my own formatter string.
I know it can be done with String.Format, but I am curious if there is a F# way with sprintf, or ksprinf; can it be done?
Simple answer
EDIT: Diego Esmerio on F# Slack showed me a simpler way that I honestly never thought of while working out the answer below. The trick is to use PrintfFormat directly, like as follows.
// Credit: Diego. This
let formatPrec precision =
PrintfFormat<float -> string,unit,string,string>(sprintf "%%1.%if" precision)
let x = 15.234
let a = sprintf (formatPrec 0) x
let b = sprintf (formatPrec 1) x
let c = sprintf (formatPrec 3) x
Output:
val formatPrec : precision:int -> PrintfFormat<(float -> string),unit,string,string>
val x : float = 15.234
val a : string = "15"
val b : string = "15.2"
val c : string = "15.234"
This approach is arguably much simpler than the Expr-based approach below. For both approaches, be careful with the formatting string, as it will compile just fine, but break at runtime if it is invalid.
Original answer (complex)
This isn't trivial to do, because functions like sprintf and printfn are compile-time special-case functions that turn your string-argument into a function (in this case of type float -> string).
There are some things you can do with kprintf, but it won't allow the formatting-argument to become a dynamic value, since the compiler still wants to type-check that.
However, using quotations we can build such function ourselves. The easy way is to create quotation from your expression and to change the parts we need to change.
The starting point is this:
> <# sprintf "%3.1f" #>
val it : Expr<(float -> string)> =
Let (clo1,
Call (None, PrintFormatToString,
[Coerce (NewObject (PrintfFormat`5, Value ("%3.1f")), PrintfFormat`4)]),
Lambda (arg10, Application (clo1, arg10)))
...
That may look like a whole lot of mess, but since we only need to change one tiny bit, we can do this rather simply:
open Microsoft.FSharp.Quotations // part of F#
open Microsoft.FSharp.Quotations.Patterns // part of F#
open FSharp.Quotations.Evaluator // NuGet package (with same name)
// this is the function that in turn will create a function dynamically
let withFormat format =
let expr =
match <# sprintf "%3.1f" #> with
| Let(var, expr1, expr2) ->
match expr1 with
| Call(None, methodInfo, [Coerce(NewObject(ctor, [Value _]), mprintFormat)]) ->
Expr.Let(var, Expr.Call(methodInfo, [Expr.Coerce(Expr.NewObject(ctor, [Expr.Value format]), mprintFormat)]), expr2)
| _ -> failwith "oops" // won't happen
| _ -> failwith "oops" // won't happen
expr.CompileUntyped() :?> (float -> string)
To use this, we can now simply do this:
> withFormat "%1.2f" 123.4567899112233445566;;
val it : string = "123.46"
> withFormat "%1.5f" 123.4567899112233445566;;
val it : string = "123.45679"
> withFormat "%1.12f" 123.4567899112233445566;;
val it : string = "123.456789911223"
Or like this:
> let format = "%0.4ef";;
val format : string = "%0.4ef"
> withFormat format 123.4567899112233445566;;
val it : string = "1.2346e+002f"
It doesn't matter whether the format string is now a fixed string during compile time. However, if this is used in performance sensitive area, you may want to cache the resulting functions, as recompiling an expression tree is moderately expensive.
EDIT: possible solutions at bottom
I'm doing some data work where I need to be very careful about string lengths that will eventually be sent in fixed width text output, stored in limited size nvarchar fields, etc. I want to have good strict typing for these rather than naked System.String types.
Suppose I've got some code like this to represent these, with a few useful module functions that play nicely with Result.map, Option.map, etc.
module String40 =
let private MaxLength = 40
type T = private T of string
let create (s:string) = checkStringLength MaxLength s |> Result.map T
let trustCreate (s:string) = checkStringLength MaxLength s |> Result.okVal |> T
let truncateCreate (s:string) = truncateStringToLength MaxLength s |> T
let toString (T s) = s
type T with
member this.AsString = this |> toString
module String100 =
let private MaxLength = 100
type T = private T of string
let create (s:string) = checkStringLength MaxLength s |> Result.map T
let trustCreate (s:string) = checkStringLength MaxLength s |> Result.okVal |> T
let truncateCreate (s:string) = truncateStringToLength MaxLength s |> T
let toString (T s) = s
type T with
member this.AsString = this |> toString
Obviously these are almost entirely repetitive with only the module name and max length different in each block.
What options are available to try and cut down on the repetitiveness here? I would love to have something like this:
type String40 = LengthLimitedString<40>
type String100 = LengthLimitedString<100>
tryToRetrieveString () // returns Result<string, ERRType>
|> Result.bind String40.create
T4 code generation doesn't seem to be an option for F# projects
Type providers seem like overkill for this kind of simple templating, and as best as I can tell they can only produce classes, not modules.
I'm aware of Scott Wlaschin's constrained strings page, but I end up with roughly the same level of repetitive code in the 'Create a type', 'Implement IWrappedString', 'create a public constructor' steps he lists.
These code blocks are fairly short and it wouldn't be the end of the world to just copy/paste a dozen times for the different field lengths. But I feel like I'm missing a simpler way to do this.
UPDATE:
One other note is that it's important that records using these types give information about what type they're carrying:
type MyRecord =
{
FirstName: String40;
LastName: String100;
}
and not be something like
type MyRecord =
{
FirstName: LimitedString;
LastName: LimitedString;
}
Tomas' answer, using the Depended Type Provider nuget library is a pretty good one, and would be a good solution for a lot of people who are fine with its behavior as-is. I felt like it would be a little tricky to extend and customize unless I wanted to maintain my own copy of a type provider which I was hoping to avoid.
Marcelo's suggestion of static parameter constraints was a fairly productive path of research. They give me basically what I was looking for -- a generic argument that is basically an 'interface' for a static methods. However the kicker is that they require inline functions to operate and I don't have the time to evaluate how much that would or would not matter in my code base.
But I took that and altered it to use regular generic constraints. It's a bit goofy to have to instantiate an object to get a max-length value, and fsharp type/generic code is just gross to look at, but from the module users's perspective it's clean, and I can easily extend this however I want.
type IMaxLengthProvider = abstract member GetMaxLength: unit -> int
type MaxLength3 () = interface IMaxLengthProvider with member this.GetMaxLength () = 3
type MaxLength4 () = interface IMaxLengthProvider with member this.GetMaxLength () = 4
module LimitedString =
type T< 'a when 'a :> IMaxLengthProvider> = private T of string
let create< 't when 't :> IMaxLengthProvider and 't : (new:unit -> 't)> (s:string) =
let len = (new 't()).GetMaxLength()
match checkStringLength len s with
| Ok s ->
let x : T< 't> = s |> T
x |> Ok
| Error e -> Error e
let trustCreate< 't when 't :> IMaxLengthProvider and 't : (new:unit -> 't)> (s:string) =
let len = (new 't()).GetMaxLength()
match checkStringLength len s with
| Ok s ->
let x : T< 't> = s |> T
x
| Error e ->
let msg = e |> formErrorMessage
failwith msg
let truncateCreate< 't when 't :> IMaxLengthProvider and 't : (new:unit -> 't)> (s:string) =
let len = (new 't()).GetMaxLength()
let s = truncateStringToLength len s
let x : T< 't> = s |> T
x
let toString (T s) = s
type T< 'a when 'a :> IMaxLengthProvider> with
member this.AsString = this |> toString
module test =
let dotest () =
let getString () = "asd" |> Ok
let mystr =
getString ()
|> Result.bind LimitedString.create<MaxLength3>
|> Result.okVal
|> LimitedString.toString
sprintf "it is %s" mystr
I think the BoundedString type provider from the Dependent type provider project lets you do exactly what you need. Using the example from the project documentation, you can do e.g.:
type ProductDescription = BoundedString<10, 2000>
type ProductName = BoundedString<5, 50>
type Product = { Name : ProductName; Description : ProductDescription }
let newProduct (name : string) (description : string) : Product option =
match ProductName.TryCreate(name), ProductDescription.TryCreate(description) with
| Some n, Some d -> { Name = n; Description = d }
| _ -> None
I don't know how many people use this project in practice, but it seems quite simple and it does exactly what you were asking for, so it might be worth a try.
Using a touch of careful reflection magic, we can achieve a lot and get some really nice types. How about something like this?
module Strings =
type [<AbstractClass>] Length(value: int) =
member this.Value = value
let getLengthInst<'L when 'L :> Length> : 'L =
downcast typeof<'L>.GetConstructor([||]).Invoke([||])
type LimitedString<'Length when 'Length :> Length> =
private | LimitedString of maxLength: 'Length * value: string
member this.Value =
let (LimitedString(_, value)) = this in value
member this.MaxLength =
let (LimitedString(maxLength, _)) = this in maxLength.Value
module LimitedString =
let checkStringLength<'L when 'L :> Length> (str: string) =
let maxLength = getLengthInst<'L>.Value
if str.Length <= maxLength then Ok str
else Error (sprintf "String of length %d exceeded max length of %d" str.Length maxLength)
let create<'L when 'L :> Length> (str: string) =
checkStringLength<'L> str
|> Result.map (fun str -> LimitedString (getLengthInst<'L>, str))
open Strings
// === Usage ===
type Len5() = inherit Length(5)
type Len1() = inherit Length(1)
// Ok
LimitedString.create<Len5> "Hello"
// Error
LimitedString.create<Len1> "world"
One option might be to use a single module for limited-length strings that uses curried parameters for the length-limit and the string itself, then just partially apply the limit parameter. An implementation might look like this:
module LimitedString =
type T = private T of string
let create length (s:string) = checkStringLength length s |> Result.map T
let trustCreate length (s:string) = checkStringLength length s |> Result.okVal |> T
let truncateCreate length (s:string) = truncateStringToLength length s |> T
let toString (T s) = s
type T with
member this.AsString = this |> toString
Then, your modules for each length would still be required, but wouldn't have all the boilerplate:
module String100 =
let create = LimitedString.create 100
let trustCreate = LimitedString.trustCreate 100
let truncateCreate = LimitedString.truncateCreate 100
EDIT
After reading the comment and the update to the original post, I would change my suggestion a bit. Instead of defining the T type inside each module, I would have a specific struct-type single-case union for each string length at the top level. Then, I would move to toString to the individual string modules. Finally, I would add one more parameter to the LimitedString module to allow us to partially apply both the length and the specific single-case union type:
[<Struct>] type String40 = private String40 of string
[<Struct>] type String100 = private String100 of string
module LimitedString =
let create length ctor (s:string) = checkStringLength length s |> Result.map ctor
let trustCreate length ctor (s:string) = checkStringLength length s |> Result.okVal |> ctor
let truncateCreate length ctor (s:string) = truncateStringToLength length s |> ctor
module String40 =
let create = LimitedString.create 40 String40
let trustCreate = LimitedString.trustCreate 40 String40
let truncateCreate = LimitedString.truncateCreate 40 String40
let toString (String40 s) = s
module String100 =
let create = LimitedString.create 100 String100
let trustCreate = LimitedString.trustCreate 100 String100
let truncateCreate = LimitedString.truncateCreate 100 String100
let toString (String100 s) = s
type MyRecord =
{
FirstName: String40
LastName: String100
}
There's still a fair amount of boilerplate here, but I think this is in the ballpark for a solution using single-case unions and modules. A Type Provider might be possible, but you would have to consider whether the added complexity outweighs the boilerplate.
Can't you use Static Parameters, as shown on the F#.Data package's HtmlProvider example?
I am trying to use F# as a REPL and scripting which uses C# library. When I evaluate an object in REPL, it prints its internal structure:
> <expression>;;
val it: <type> =
<subtype> {<prop> = <value>;
...
<prop> = <value>;}
Then I am writing a script with the same expression and want it to print same output. But I cannot find any print function which would do it. The closest I could find is printfn "%O" which uses ToString() method, which is not defined in my case and just prints the object type.
This seems to be a simple question but I cannot find it answered here or anywhere in Google.
How to generate the F# type signature similar to FSI in my own code? seems to be focused on type, and I basically need rather pretty-printed value.
PS: seems like it is code which is internal to fsi.exe. See fsi.fs and sformat.fs sources. I tried to invoke them through reflection, but simple Internal.Utilities.StructuredFormat.any_to_string(value) printed just a type. Would still be nice if anybody knows how to invoke it correctly, but for time being I decided not to spend more efforts on it.
I just had the same issue, but in my case, printfn "%A" gave exactly the same result as what I see in F# Interactive (bar the indentation):
For a list:
> let l = [(2,"a")];;
val l : (int * string) list = [(2, "a")]
> printfn "%A" l;;
[(2, "a")]
For a record:
> type R = { A: string; B: int };;
type R =
{A: string;
B: int;}
> let r = { A = "Foo"; B = 1 };;
val r : R = {A = "Foo";
B = 1;}
> printfn "%A" r;;
{A = "Foo";
B = 1;}
For a non-F# datatype:
> let u = UriBuilder("http", "bar", 80);;
val u : UriBuilder = http://bar:80/
> printfn "%A" u;;
http://bar:80/
There's information out there on how to do custom processing on a format and its parts. I want to do something a bit simpler, specifically, I want to do something to the effect of:
let writelog : string -> unit = ... // write the string to the log
let writelogf = sprintf >> writelog // write a formatted string to the log
I'm not too surprised that the compiler gets confused by this, but is there any way to get it to work?
The simplest way to define your own function that takes a formatting string like printf is to use Printf.kprintf function. The first argument of kprintf is a function that is used to display the resulting string after formatting (so you can pass it your writelog function):
let writelog (s:string) = printfn "LOG: %s" s
let writelogf fmt = Printf.kprintf writelog fmt
The fmt parameter that is passed as a second argument is the special format string. This works better than jpalmer's solution, because if you specify some additional arguments, they will be directly passed to kprintf (so the number of arguments can depend on the formatting string).
You can write:
> writelogf "Hello";;
LOG: Hello
> writelogf "Hello %d" 42;;
LOG: Hello 42
This works
> let writelog = fun (s:string) -> printfn "%s" s;;
val writelog : string -> unit
> let writelogf arg = sprintf arg >> writelog;;
val writelogf : Printf.StringFormat<('a -> string)> -> ('a -> unit)
> writelogf "hello %s" "world";;
hello world
val it : unit = ()
>
(session is from FSI)
key is in the extra argument to writelogf
I have byte array as input. I would like to convert that array to string that contains hexadecimal representation of array values. This is F# code:
let ByteToHex bytes =
bytes
|> Array.map (fun (x : byte) -> String.Format("{0:X2}", x))
let ConcatArray stringArray = String.Join(null, (ByteToHex stringArray))
This produces result I need, but I would like to make it more compact so that I have only one function.
I could not find function that would concat string representation of each byte at the end
of ByteToHex.
I tried Array.concat, concat_map, I tried with lists, but the best I could get is array or list of strings.
Questions:
What would be simplest, most elegant way to do this?
Is there string formatting construct in F# so that I can replace String.Format from System assembly?
Example input: [|0x24uy; 0xA1uy; 0x00uy; 0x1Cuy|] should produce string "24A1001C"
There is nothing inherently wrong with your example. If you'd like to get it down to a single expression then use the String.contcat method.
let ByteToHex bytes =
bytes
|> Array.map (fun (x : byte) -> System.String.Format("{0:X2}", x))
|> String.concat System.String.Empty
Under the hood, String.concat will just call into String.Join. Your code may have to be altered slighly though because based on your sample you import System. This may create a name resolution conflict between F# String and System.String.
If you want to transform and accumulate in one step, fold is your answer. sprintf is the F# string format function.
let ByteToHex (bytes:byte[]) =
bytes |> Array.fold (fun state x-> state + sprintf "%02X" x) ""
This can also be done with a StringBuilder
open System.Text
let ByteToHex (bytes:byte[]) =
(StringBuilder(), bytes)
||> Array.fold (fun state -> sprintf "%02X" >> state.Append)
|> string
produces:
[|0x24uy; 0xA1uy; 0x00uy; 0x1Cuy|] |> ByteToHex;;
val it : string = "24A1001C"
Here's another answer:
let hashFormat (h : byte[]) =
let sb = StringBuilder(h.Length * 2)
let rec hashFormat' = function
| _ as currIndex when currIndex = h.Length -> sb.ToString()
| _ as currIndex ->
sb.AppendFormat("{0:X2}", h.[currIndex]) |> ignore
hashFormat' (currIndex + 1)
hashFormat' 0
The upside of this one is that it's tail-recursive and that it pre-allocates the exact amount of space in the string builder as will be required to convert the byte array to a hex-string.
For context, I have it in this module:
module EncodingUtils
open System
open System.Text
open System.Security.Cryptography
open Newtonsoft.Json
let private hmacmd5 = new HMACMD5()
let private encoding = System.Text.Encoding.UTF8
let private enc (str : string) = encoding.GetBytes str
let private json o = JsonConvert.SerializeObject o
let md5 a = a |> (json >> enc >> hmacmd5.ComputeHash >> hashFormat)
Meaning I can pass md5 any object and get back a JSON hash of it.
Here's another. I'm learning F#, so feel free to correct me with more idiomatic ways of doing this:
let bytesToHexString (bytes : byte[]) : string =
bytes
|> Seq.map (fun c -> c.ToString("X2"))
|> Seq.reduce (+)
Looks fine to me. Just to point out another, in my opinion, very helpful function in the Printf module, have a look at ksprintf. It passes the result of a formated string into a function of your choice (in this case, the identity function).
val ksprintf : (string -> 'd) -> StringFormat<'a,'d> -> 'a
sprintf, but call the given 'final' function to generate the result.
To be honest, that doesn't look terrible (although I also have very little F# experience). Does F# offer an easy way to iterate (foreach)? If this was C#, I might use something like (where raw is a byte[] argument):
StringBuilder sb = new StringBuilder();
foreach (byte b in raw) {
sb.Append(b.ToString("x2"));
}
return sb.ToString()
I wonder how that translates to F#...