I have a recursive function that contains a series of matches that either make the recursive call back to the function, or make a call to failwith.
This is basically a hybrid implementation of the recursive descent parser descibed in Don Syme's Expert F# book (page 180) and the parsing example shown here: http://fsharpforfunandprofit.com/posts/pattern-matching-command-line/
Here is a snippet of my own code.
let rec parseTokenListRec tokenList optionsSoFar =
match tokenList with
| [] -> optionsSoFar
| SOURCE::t ->
match t with
| VALUE x::tt -> parseTokenListRec (returnNonValueTail t) {optionsSoFar with Source = (returnConcatHeadValues t)}
| _ -> failwith "Expected a value after the source argument."
| REGISTRY::t ->
...
A full code listing can be found at http://fssnip.net/nU
The way the code is currently written, when the function has finished working its way through the tokenList, it will return the optionsSoFar record that has been compiled via the object expression {optionsSoFar with Source = (returnConcatHeadValues t)}, or it will throw an exception if an invalid argument is found.
I want to refactor this so that the function does not rely on an exception, but will always return a value of some sort that can be handled by the calling function. The idea I have is to return a discriminated union rather than a record.
This discriminated union would be something like
type Result =
|Success of Options
|Failure of string
The problem I had when I tried to refactor the code was that I couldn't figure out how to get the success value of the DU to initialize via an object expression. Is this possible?
The examples I've looked at on MSDN (http://msdn.microsoft.com/en-us/library/vstudio/dd233237(v=vs.100).aspx), fsharpforfunandprofit (http://fsharpforfunandprofit.com/posts/discriminated-unions/) and elsewhere haven't quite cleared this up for me.
I'm worried that I'm not making any sense here. I'm happy to clarify if needed.
If I understand it correctly, in you current solution, the type of optionsSoFar is Options. The code becomes trickier if you change the type of optionsSoFar to your newly defined Result.
However, I think you do not need to do that - you can keep optionsSoFar : Options and change the function to return Result. This works because you never need to call the function recursively after it fails:
let rec parseTokenListRec tokenList optionsSoFar =
match tokenList with
| [] -> Success optionsSoFar
| SOURCE::t ->
match t with
| VALUE x::tt ->
{optionsSoFar with Source = (returnConcatHeadValues t)}
|> parseTokenListRec (returnNonValueTail t)
| _ -> Failure "Expected a value after the source argument."
| REGISTRY::t -> ...
If you actually wanted to update Source in a Result value, then I'd probably write something like:
module Result =
let map f = function
| Success opt -> f opt
| Failure msg -> Failure msg
Then you could write a transformation as follows:
resultSoFar
|> Result.map (fun opts -> {opts with Source = returnConcatHeadValues t})
|> parseTokenListRec (returnNonValueTail t)
Related
I just started programming functionally. My current tiny project to start would be a basic pokemon battle.
Code first, explanation follows.
let choosePokemon () =
let mutable pokemon = DemoData.schiggy
let msg = Console.ReadLine()
match msg with
| "Schiggy" -> pokemon <- DemoData.schiggy
| "Pikachu" -> pokemon <- DemoData.pikachu
| "Kleinstein" -> pokemon <- DemoData.kleinstein
| "Karpador" -> pokemon <- DemoData.karpador
pokemon
I am currently asking people to enter the name of a Pokemon and if it matches a predefined set (Schiggy, Pikachu, Kleinstein or Karpador) it gives them the respective Pokemon. Alternatively it gives them the default Pokemon.
I am currently creating it before matching it with "let mutable pokemon = DemoData.schiggy". I don't want that. I just want to assign it based on its name.
If I could go without that line I'd avoid making it mutable, which is something I don't want anyways.
Additional question further down the line: When Pokemon attack each other their hp will decrease.
How can I avoid using a mutable int when facing a changing int value?
Thanks in advance :)
Just return the result from the match - there's no need to declare a variable:
let choosePokemon () =
let msg = Console.ReadLine()
match msg with
| "Schiggy" -> DemoData.schiggy
| "Pikachu" -> DemoData.pikachu
| "Kleinstein" -> DemoData.kleinstein
| "Karpador" -> DemoData.karpador
The function is still impure, though, since it performs I/O (Console.ReadLine)...
It's also partial, since it'll crash on any other input than the four strings being matched. Even misspellings and case mistakes are going to throw an exception.
A more robust pure function would be something like this:
let choosePokemon (input : string) =
match input.Trim().ToUpperInvariant () with
| "PIKACHU" -> DemoData.pikachu
| "KLEINSTEIN" -> DemoData.kleinstein
| "KARPADOR" -> DemoData.karpador
| _ -> DemoData.schiggy
Contrary to the OP that performs I/O, this version is a pure function, since there's no non-deterministic behaviour. It's also total, since it returns schiggy as a default value.
To get the behaviour where you ask the user to input a value, compose the impure Console.ReadLine action with the pure choosePokemon function:
Console.ReadLine >> choosePokemon
let memoize (sequence: seq<'a>) =
let cache = Dictionary()
seq {for i in sequence ->
match cache.TryGetValue i with
| true, v -> printf "cached"
| false,_ -> cache.Add(i ,i)
}
I will call my memoize function inside this function :
let isCached (input:seq<'a>) : seq<'a> = memoize input
If the given sequence item is cached it should print cached otherwise it will continue to add sequence value to cache.
Right now I have problems with types.
When I try to call my function like this :
let seq1 = seq { 1 .. 10 }
isCached seq1
It throws an error
"The type int does not match the type unit"
I want my function to work generic even though I return printfn. Is it possible to achieve that? And while adding value to the cache is it appropriate to give the same value to tuple?
eg:
| false,_ -> cache.Add(i ,i)
I think the problem is that your memoize function does not actually return the item from the source sequence as a next element of the returned sequence. Your version only adds items to the cache, but then it returns unit. You can fix that by writing:
let memoize (sequence: seq<'a>) =
let cache = Dictionary()
seq {for i in sequence do
match cache.TryGetValue i with
| true, v -> printf "cached"
| false,_ -> cache.Add(i ,i)
yield i }
I used explicit yield rather than -> because I think that makes the code more readable. With this change, the code runs as expected for me.
Tomas P beat me to the punch, but I'll post this up anyway just in case it helps.
I'm not too sure what you are trying to achieve here, but I'll say a few things that I think might help.
Firstly, the type error. Your isCached function is defined as taking a seq of type 'a, and returning a seq of type 'a. As written in your question, right now it takes a seq of type 'a, and returns a sequence of type unit. If you try modifying the output specification to seq<'b> (or actually just omitting it altogether and letting type inference do it), you should overcome the type error. This probably still won't do what you want, since you aren't actually returning the cache from that function (you can just add cache as the final line to return it). Thus, try something like:
let memoize (sequence: seq<'a>) =
let cache = Dictionary()
for i in sequence do
match cache.TryGetValue i with
| true, v -> printf "cached"
| false,_ -> cache.Add(i ,i)
cache
let isCached (input:seq<'a>) : seq<'b> = memoize input
All this being said, if you are expecting to iterate over the same sequence a lot, it might be best just to use the library function Seq.cache.
Finally, with regards to using the value as the key in the dictionary... There's nothing stopping you from doing that, but it's really fairly pointless. If you already have a value, then you shouldn't need to look it up in the dictionary. If you are just trying to memoize the sequence, then use the index of the given element as the key. Or use the specific input as the key and the output from that input as the value.
Previously asked similar question but somehow I'm not finding my way out, attempting again with another example.
The code as a starting point (a bit trimmed) is available at https://ideone.com/zkQcIU.
(it has some issue recognizing Microsoft.FSharp.Core.Result type, not sure why)
Essentially all operations have to be pipelined with the previous function feeding the result to the next one. The operations have to be async and they should return error to the caller in case an exception occurred.
The requirement is to give the caller either result or fault. All functions return a Tuple populated with either Success type Article or Failure with type Error object having descriptive code and message returned from the server.
Will appreciate a working example around my code both for the callee and the caller in an answer.
Callee Code
type Article = {
name: string
}
type Error = {
code: string
message: string
}
let create (article: Article) : Result<Article, Error> =
let request = WebRequest.Create("http://example.com") :?> HttpWebRequest
request.Method <- "GET"
try
use response = request.GetResponse() :?> HttpWebResponse
use reader = new StreamReader(response.GetResponseStream())
use memoryStream = new MemoryStream(Encoding.UTF8.GetBytes(reader.ReadToEnd()))
Ok ((new DataContractJsonSerializer(typeof<Article>)).ReadObject(memoryStream) :?> Article)
with
| :? WebException as e ->
use reader = new StreamReader(e.Response.GetResponseStream())
use memoryStream = new MemoryStream(Encoding.UTF8.GetBytes(reader.ReadToEnd()))
Error ((new DataContractJsonSerializer(typeof<Error>)).ReadObject(memoryStream) :?> Error)
Rest of the chained methods - Same signature and similar bodies. You can actually reuse the body of create for update, upload, and publish to be able to test and compile code.
let update (article: Article) : Result<Article, Error>
// body (same as create, method <- PUT)
let upload (article: Article) : Result<Article, Error>
// body (same as create, method <- PUT)
let publish (article: Article) : Result<Article, Error>
// body (same as create, method < POST)
Caller Code
let chain = create >> Result.bind update >> Result.bind upload >> Result.bind publish
match chain(schemaObject) with
| Ok article -> Debug.WriteLine(article.name)
| Error error -> Debug.WriteLine(error.code + ":" + error.message)
Edit
Based on the answer and matching it with Scott's implementation (https://i.stack.imgur.com/bIxpD.png), to help in comparison and in better understanding.
let bind2 (switchFunction : 'a -> Async<Result<'b, 'c>>) =
fun (asyncTwoTrackInput : Async<Result<'a, 'c>>) -> async {
let! twoTrackInput = asyncTwoTrackInput
match twoTrackInput with
| Ok s -> return! switchFunction s
| Error err -> return Error err
}
Edit 2 Based on F# implementation of bind
let bind3 (binder : 'a -> Async<Result<'b, 'c>>) (asyncResult : Async<Result<'a, 'c>>) = async {
let! result = asyncResult
match result with
| Error e -> return Error e
| Ok x -> return! binder x
}
Take a look at the Suave source code, and specifically the WebPart.bind function. In Suave, a WebPart is a function that takes a context (a "context" is the current request and the response so far) and returns a result of type Async<context option>. The semantics of chaining these together are that if the async returns None, the next step is skipped; if it returns Some value, the next step is called with value as the input. This is pretty much the same semantics as the Result type, so you could almost copy the Suave code and adjust it for Result instead of Option. E.g., something like this:
module AsyncResult
let bind (f : 'a -> Async<Result<'b, 'c>>) (a : Async<Result<'a, 'c>>) : Async<Result<'b, 'c>> = async {
let! r = a
match r with
| Ok value ->
let next : Async<Result<'b, 'c>> = f value
return! next
| Error err -> return (Error err)
}
let compose (f : 'a -> Async<Result<'b, 'e>>) (g : 'b -> Async<Result<'c, 'e>>) : 'a -> Async<Result<'c, 'e>> =
fun x -> bind g (f x)
let (>>=) a f = bind f a
let (>=>) f g = compose f g
Now you can write your chain as follows:
let chain = create >=> update >=> upload >=> publish
let result = chain(schemaObject) |> Async.RunSynchronously
match result with
| Ok article -> Debug.WriteLine(article.name)
| Error error -> Debug.WriteLine(error.code + ":" + error.message)
Caution: I haven't been able to verify this code by running it in F# Interactive, since I don't have any examples of your create/update/etc. functions. It should work, in principle — the types all fit together like Lego building blocks, which is how you can tell that F# code is probably correct — but if I've made a typo that the compiler would have caught, I don't yet know about it. Let me know if that works for you.
Update: In a comment, you asked whether you need to have both the >>= and >=> operators defined, and mentioned that you didn't see them used in the chain code. I defined both because they serve different purposes, just like the |> and >> operators serve different purposes. >>= is like |>: it passes a value into a function. While >=> is like >>: it takes two functions and combines them. If you would write the following in a non-AsyncResult context:
let chain = step1 >> step2 >> step3
Then that translates to:
let asyncResultChain = step1AR >=> step2AR >=> step3AR
Where I'm using the "AR" suffix to indicate versions of those functions that return an Async<Result<whatever>> type. On the other hand, if you had written that in a pass-the-data-through-the-pipeline style:
let result = input |> step1 |> step2 |> step3
Then that would translate to:
let asyncResult = input >>= step1AR >>= step2AR >>= step3AR
So that's why you need both the bind and compose functions, and the operators that correspond to them: so that you can have the equivalent of either the |> or the >> operators for your AsyncResult values.
BTW, the operator "names" that I picked (>>= and >=>), I did not pick randomly. These are the standard operators that are used all over the place for the "bind" and "compose" operations on values like Async, or Result, or AsyncResult. So if you're defining your own, stick with the "standard" operator names and other people reading your code won't be confused.
Update 2: Here's how to read those type signatures:
'a -> Async<Result<'b, 'c>>
This is a function that takes type A, and returns an Async wrapped around a Result. The Result has type B as its success case, and type C as its failure case.
Async<Result<'a, 'c>>
This is a value, not a function. It's an Async wrapped around a Result where type A is the success case, and type C is the failure case.
So the bind function takes two parameters:
a function from A to an async of (either B or C)).
a value that's an async of (either A or C)).
And it returns:
a value that's an async of (either B or C).
Looking at those type signatures, you can already start to get an idea of what the bind function will do. It will take that value that's either A or C, and "unwrap" it. If it's C, it will produce an "either B or C" value that's C (and the function won't need to be called). If it's A, then in order to convert it to an "either B or C" value, it will call the f function (which takes an A).
All this happens within an async context, which adds an extra layer of complexity to the types. It might be easier to grasp all this if you look at the basic version of Result.bind, with no async involved:
let bind (f : 'a -> Result<'b, 'c>) (a : Result<'a, 'c>) =
match a with
| Ok val -> f val
| Error err -> Error err
In this snippet, the type of val is 'a, and the type of err is 'c.
Final update: There was one comment from the chat session that I thought was worth preserving in the answer (since people almost never follow chat links). Developer11 asked,
... if I were to ask you what Result.bind in my example code maps to your approach, can we rewrite it as create >> AsyncResult.bind update? It worked though. Just wondering i liked the short form and as you said they have a standard meaning? (in haskell community?)
My reply was:
Yes. If the >=> operator is properly written, then f >=> g will always be equivalent to f >> bind g. In fact, that's precisely the definition of the compose function, though that might not be immediately obvious to you because compose is written as fun x -> bind g (f x) rather than as f >> bind g. But those two ways of writing the compose function would be exactly equivalent. It would probably be very instructive for you to sit down with a piece of paper and draw out the function "shapes" (inputs & outputs) of both ways of writing compose.
Why do you want to use Railway Oriented Programming here? If you just want to run a sequence of operations and return information about the first exception that occurs, then F# already provides a language support for this using exceptions. You do not need Railway Oriented Programming for this. Just define your Error as an exception:
exception Error of code:string * message:string
Modify the code to throw the exception (also note that your create function takes article but does not use it, so I deleted that):
let create () = async {
let ds = new DataContractJsonSerializer(typeof<Error>)
let request = WebRequest.Create("http://example.com") :?> HttpWebRequest
request.Method <- "GET"
try
use response = request.GetResponse() :?> HttpWebResponse
use reader = new StreamReader(response.GetResponseStream())
use memoryStream = new MemoryStream(Encoding.UTF8.GetBytes(reader.ReadToEnd()))
return ds.ReadObject(memoryStream) :?> Article
with
| :? WebException as e ->
use reader = new StreamReader(e.Response.GetResponseStream())
use memoryStream = new MemoryStream(Encoding.UTF8.GetBytes(reader.ReadToEnd()))
return raise (Error (ds.ReadObject(memoryStream) :?> Error)) }
And then you can compose functions just by sequencing them in async block using let! and add exception handling:
let main () = async {
try
let! created = create ()
let! updated = update created
let! uploaded = upload updated
Debug.WriteLine(uploaded.name)
with Error(code, message) ->
Debug.WriteLine(code + ":" + message) }
If you wanted more sophisticated exception handling, then Railway Oriented Programming might be useful and there is certainly a way of integrating it with async, but if you just want to do what you described in your question, then you can do that much more easily with just standard F#.
How can I create a function called getFuncName that takes a function of type (unit -> 'a) and returns its name.
I was talking to one of the C# devs and they said you could use the .Method property on a Func type as shown in an example here.
I tried to convert this to F# :
for example convert (unit -> 'a) to a type Func<_> then call the property on it but it always returns the string "Invoke".
let getFuncName f =
let fFunc = System.Func<_>(fun _ -> f())
fFunc.Method.Name
let customFunc() = 1.0
// Returns "Invoke" but I want it to return "customFunc"
getFuncName customFunc
A bit of background to this problem is:
I have created an array of functions of type (unit -> Deedle.Frame). I now want to cycle through those functions invoking them and saving them to csv with the csv name having the same name as the function. Some hypothetical code is below:
let generators : (unit -> Frame<int, string>) array = ...
generators
|> Array.iter (fun generator -> generator().SaveCsv(sprintf "%s\%s.csv" __SOURCE_DIRECTORY__ (getFuncName generator)))
This is being used in a scripting sense rather than as application code.
Not sure how you searched for information, but the first query to the search engine gave me this response:
let getFuncName f =
let type' = f.GetType()
let method' = type'.GetMethods() |> Array.find (fun m -> m.Name="Invoke")
let il = method'.GetMethodBody().GetILAsByteArray()
let methodCodes = [byte OpCodes.Call.Value;byte OpCodes.Callvirt.Value]
let position = il |> Array.findIndex(fun x -> methodCodes |> List.exists ((=)x))
let metadataToken = BitConverter.ToInt32(il, position+1)
let actualMethod = type'.Module.ResolveMethod metadataToken
actualMethod.Name
Unfortunately, this code only works when F# compiler does not inline function body into calling method.
Taken from here
Although there may be a more simple way.
I need to pass two list as command line arguments in ocaml.
I used the following code to access it in the program.
let list1=Sys.argv.(1);;
let list2=Sys.argv.(2);;
I need to have the list1 and list2 as list of integers.
I am getting the error
This expression has type string but an expression was expected of type
int list
while processing.
How can I convert that arguments to a list of integers.
The arguments are passed in this format [1;2;3;4] [1;5;6;7]
Sys.argv.(n) will always be a string. You need to parse the string into a list of integers. You could try something like this:
$ ocaml
OCaml version 4.01.0
# #load "str.cma";;
# List.map int_of_string (Str.split (Str.regexp "[^0-9]+") "[1;5;6;7]");;
- : int list = [1; 5; 6; 7]
Of course this doesn't check the input for correct form. It just pulls out sequences of digits by brute force. To do better you need to do some real lexical analysis and simple parsing.
(Maybe this is obvious, but you could also test your function in the toplevel (the OCaml read-eval-print loop). The toplevel will handle the work of making a list from what you type in.)
As Sys.argv is a string array, you need to write your own transcription function.
I guess the simplest way to do this is to use the Genlex module provided by the standard library.
let lexer = Genlex.make_lexer ["["; ";"; "]"; ]
let list_of_string s =
let open Genlex in
let open Stream in
let stream = lexer (of_string s) in
let fail () = failwith "Malformed string" in
let rec aux acc =
match next stream with
| Int i ->
( match next stream with
| Kwd ";" -> aux (i::acc)
| Kwd "]" -> i::acc
| _ -> fail () )
| Kwd "]" -> acc
| _ -> fail ()
in
try
match next stream with
| Kwd "[" -> List.rev (aux [])
| _ -> fail ()
with Stream.Failure -> fail ()
let list1 = list_of_string Sys.argv.(1)
let list2 = list_of_string Sys.argv.(2)
Depending on the OCaml flavor you want to use, some other library may look more interesting. If you like yacc, Menhir may solve your problem in a few lines of code.