I have written this F# code:
let tuples = [|("A",1,0);("A",2,3);("A",3,6)|]
let tupleSubset =
tuples
|> Seq.map(fun values ->
values.[0],
values.[2])
printfn "%A" tupleSubset
I am getting: The operator 'expr.[idx]' has been used on an object of indeterminate type based on information prior to this program point. Consider adding further type constraints
Can anyone tell me what I am doing wrong?
A simple way around the error you are getting would be the following:
let tuples = [|("A",1,0); ("A",2,3); ("A",3,6)|]
let tupleSubset = tuples |> Seq.map (fun (a, b, c) -> a, b)
printfn "%A" tupleSubset
As to why you are getting the error: note that the kind of index dereferencing you are attempting with values.[0] and values.[1] works for arrays, dictionaries, etc., but not for tuples, whereas each values has the type string * int * int.
Since you don't need the third element of the tuple, you can even choose not to bind it to a symbol, by writing the second line as follows:
let tupleSubset = tuples |> Seq.map (fun (a, b, _) -> a, b)
The explanation of Shredderoy is correct, accessing an element by its index is reserved to arrays, which are contiguous blocks of memory.
For tuple, if you want to access the values of a tuple in F# the general method is to pattern match
let tuple = a,b,c
let (x,y,z) = tuple
In the case of pair, the function fst and snd also give a similar access
PS : In you initial array, you dont need to use parenthesis, as the compiler recognizes the , as a tuple
Related
I have a text file that contains the following and I need to retrieve the value assigned to taskId, which in this case is AWc34YBAp0N7ZCmVka2u.
projectKey=ProjectName
serverUrl=http://localhost:9090
serverVersion=10.5.32.3
strong text**interfaceUrl=http://localhost:9090/interface?id=ProjectName
taskId=AWc34YBAp0N7ZCmVka2u
taskUrl=http://localhost:9090/api/ce/task?id=AWc34YBAp0N7ZCmVka2u
I have two different ways of reading the file that I've wrote.
let readLines (filePath:string) = seq {
use sr = new StreamReader (filePath)
while not sr.EndOfStream do
yield sr.ReadLine ()
}
readLines (FindFile currentDirectory "../**/sample.txt")
|> Seq.iter (fun line ->
printfn "%s" line
)
and
let readLines (filePath:string) =
(File.ReadAllLines filePath)
readLines (FindFile currentDirectory "../**/sample.txt")
|> Seq.iter (fun line ->
printfn "%s" line
)
At this point, I don't know how to approach getting the value I need. Options that, I think, are on the table are:
use Contains()
Regex
Record type
Active Pattern
How can I get this value returned and fail if it doesn't exist?
I think all the options would be reasonable - it depends on how complex the file will actually be. If there is no escaping then you can probably just look for = in the line and use that to split the line into a key value pair. If the syntax is more complex, this might not always work though.
My preferred method would be to use Split on string - you can then filter to find values with your required key, map to get the value and use Seq.head to get the value:
["foo=bar"]
|> Seq.map (fun line -> line.Split('='))
|> Seq.filter (fun kvp -> kvp.[0] = "foo")
|> Seq.map (fun kvp -> kvp.[1])
|> Seq.head
Using active patterns, you could define a pattern that takes a string and splits it using = into a list:
let (|Split|) (s:string) = s.Split('=') |> List.ofSeq
This then lets you get the value using Seq.pick with a pattern matching that looks for strings where the substring before = is e.g. foo:
["foo=bar"] |> Seq.pick (function
| Split ["foo"; value] -> Some value
| _ -> None)
The active pattern trick is quite neat, but it might be unnecessarily complicating the code if you only need this in one place.
No problem here:
module Seq =
let private rnd = Random Environment.TickCount
let random =
fun (items : 'T seq) ->
let count = Seq.length items
items |> Seq.nth (rnd.Next count)
The signature of Seq.random is items:seq<'T> -> 'T. All good.
Yes, I know that I could just let random items = [...], that is not the point.
The point is that items is suddenly constrained to be type seq<obj> when I do this:
module Seq =
let random =
let rnd = Random Environment.TickCount
fun (items : 'T seq) ->
let count = Seq.length items
items |> Seq.nth (rnd.Next count)
... i.e. I add the Random object as a closure. If I hover over random, Intellisense shows me that the signature has become items:seq<obj> -> obj.
Interestingly, if I select the code and hit [Alt]+[Enter] to execute it in F# Interactive, the signature shows as seq<'a> -> 'a. WTH??
So, what's going on, here? Why the confusion and inconsistency in type inference?
This is due to the so-called Value Restriction. Cutting a long story short, syntactical values cannot be generic, because it might break things when mutations occur, and the compiler cannot always reliably prove immutability. (note that, even though random is a function semantically, it is still a value syntactically, and that's what matters)
But sometimes the compiler can prove immutability. This is why your first example works: when the right side of a let is a straight up lambda expression, the compiler can tell with certainty that it is immutable, and so it lets this pass.
Another example would be let x = [] - here the compiler can see that the nil list [] is immutable. On the other hand, let x = List.append [] [] won't work, because the compiler can't prove immutability in that case.
This "relaxation" of value restriction is done in F# on a case-by-case basis. F# compiler only goes as far as to handle a few special cases: literals, lambda expressions, etc., but it doesn't have a full-fledged mechanism for proving immutability in general. This is why, once you step outside of those special cases, you're not allowed to have generic values.
You can technically defeat this by adding explicit type arguments. Logically, this tells the compiler "Yes, I know it's a generic value, and that's what I meant for it to be".
let random<'t> : seq<'t> -> 't =
let rnd = Random Environment.TickCount
fun items ->
let count = Seq.length items
items |> Seq.nth (rnd.Next count)
let x = random [1;2;3]
But this will still not do what you want, because behind the scenes, such definition will be compiled to a parameterless generic method, and every time you reference such "value", the method will be called and return you a new function - with a brand new rnd baked in for every call. In other words, the above code will be equivalent to this:
let random() =
let rnd = Random Environment.TickCount
fun items ->
let count = Seq.length items
items |> Seq.nth (rnd.Next count)
let x = random() [1;2;3]
I have a Dictionary over which I initially iterated thusly:
myDictionary |> Seq.iter (fun kvp -> doSomething kvp.Key kvp.Value)
Later, I discovered that I could make use of the KeyValue active pattern, and do this:
myDictionary |> Seq.iter (fun (KeyValue (k, v)) -> doSomething k v)
Knowing that active patterns aren't some form of preprocessor directive, how am I able to substitute the kvp argument in the lambda for a function that decomposes it?
Functions arguments call always be destructured using pattern matching. For instance:
let getSingleton = fun [x] -> x
let getFirst = fun (a,b) -> a
let failIfNotOne = fun 1 -> ()
let failIfNeitherOne = fun (x,1 | 1,x) -> ()
Semantically, fun<pat>-><body> is roughly equivalent to
fun x -> match x with |<pat>-><body>
| _ -> raise MatchFailureException(...)
I think the answer from #kvb covers in enough details why you can use patterns in the arguments of fun. This is not an ad-hoc feature - in F#, you can use patterns anywhere where you can bind a variable. To show some of the examples by #kvb in another contexts:
// When declaring normal functions
let foo [it] = it // Return the value from a singleton list
let fst (a, b) = a // Return first element of a pair
// When assigning value to a pattern using let
let [it] = list
let (a, b) = pair
Similarly, you can use patterns when writing fun. The match construct is a bit more powerful, because you can specify multiple clauses.
Now, active patterns are not really that magical. They are just normal functions with special names. The compiler searches for active patterns in scope when it finds a named pattern. For example, the pattern you're using is just a function:
val (|KeyValue|) : KeyValuePair<'a,'b> -> 'a * 'b
The pattern turns a KevValuePair object into a normal F# tuple that is then matched by a nested pattern (k, v) (which assigns the first element to k and the second to v). The compiler essentially translates your code to:
myDictionary |> Seq.iter (fun _arg0 ->
let _arg1 = (|KeyValue|) _arg0
let (k, v) = _arg1
doSomething k v )
I'm reading Expert F# book and I found this code
open System.Collections.Generic
let divideIntoEquivalenceClasses keyf seq =
// The dictionary to hold the equivalence classes
let dict = new Dictionary<'key,ResizeArray<'T>>()
// Build the groupings
seq |> Seq.iter (fun v ->
let key = keyf v
let ok,prev = dict.TryGetValue(key)
if ok then prev.Add(v)
else let prev = new ResizeArray<'T>()
dict.[key] <- prev
prev.Add(v))
dict |> Seq.map (fun group -> group.Key, Seq.readonly group.Value)
and the example use:
> divideIntoEquivalenceClasses (fun n -> n % 3) [ 0 .. 10 ];;
val it : seq<int * seq<int>>
= seq [(0, seq [0; 3; 6; 9]); (1, seq [1; 4; 7; 10]); (2, seq [2; 5; 8])]
first for me this code is really ugly, even if this is safe, It looks more similar to imperative languages than to functional lang..specially compared to clojure. But the problem is not this...I'm having problems with the Dictionary definition
when I type this:
let dict = new Dictionary<'key,ResizeArray<'T>>();;
I get this:
pruebafs2a.fs(32,5): error FS0030: Value restriction. The value 'dict' has been inferred to have generic type
val dict : Dictionary<'_key,ResizeArray<'_T>> when '_key : equality
Either define 'dict' as a simple data term, make it a function with explicit arguments or, if you do not intend for it to be generic, add a type annotation.
is It ok?...
thanks so much
improve question:
Ok I've been reading about value restriction and I found this helpfull information
In particular, only function definitions and simple immutable data
expressions are automatically generalized
...ok..this explains why
let dict = new Dictionary<'key,ResizeArray<'T>>();;
doesn't work...and show 4 different techniques, although in my opinion they only resolve the error but aren't solutions for use generic code:
Technique 1: Constrain Values to Be Nongeneric
let empties : int list [] = Array.create 100 []
Technique 3: Add Dummy Arguments to Generic Functions When Necessary
let empties () = Array.create 100 []
let intEmpties : int list [] = empties()
Technique 4: Add Explicit Type Arguments When Necessary (similar to tec 3)
let emptyLists = Seq.init 100 (fun _ -> [])
> emptyLists<int>;;
val it : seq<int list> = seq [[]; []; []; []; ...]
----- and the only one than let me use real generic code ------
Technique 2: Ensure Generic Functions Have Explicit Arguments
let mapFirst = List.map fst //doesn't work
let mapFirst inp = List.map fst inp
Ok, in 3 of 4 techniques I need resolve the generic code before can work with this...now...returning to book example...when the compile knows the value for 'key and 'T
let dict = new Dictionary<'key,ResizeArray<'T>>()
in the scope the code is very generic for let key be any type, the same happen with 'T
and the biggest dummy question is :
when I enclose the code in a function (technique 3):
let empties = Array.create 100 [] //doesn't work
let empties () = Array.create 100 []
val empties : unit -> 'a list []
I need define the type before begin use it
let intEmpties : int list [] = empties()
for me (admittedly I'm a little dummy with static type languages) this is not real generic because it can't infer the type when I use it, I need define the type and then pass values (not define its type based in the passed values) exist other way define type without be so explicit..
thanks so much..really appreciate any help
This line
let dict = new Dictionary<'key,ResizeArray<'T>>();;
fails because when you type the ;; the compiler doesn't know what 'key and 'T are. As the error message states you need to add a type annotation, or allow the compiler to infer the type by using it later or make it a function
Examples
Type annotation change
let dict = new Dictionary<int,ResizeArray<int>>();;
Using types later
let dict = new Dictionary<'key,ResizeArray<'T>>()
dict.[1] <- 2
using a function
let dict() = new Dictionary<'key,ResizeArray<'T>>();;
This actually doesn't cause an issue when it's defined all together. That is, select the entire block that you posted and send it to FSI in one go. I get this:
val divideIntoEquivalenceClasses :
('T -> 'key) -> seq<'T> -> seq<'key * seq<'T>> when 'key : equality
However, if you type these individually into FSI then as John Palmer says there is not enough information in that isolated line for the interpreter to determine the type constraints. John's suggestions will work, but the original code is doing it correctly - defining the variable and using it in the same scope so that the types can be inferred.
for me this code is really ugly, even if this is safe, It looks more similar to imperative languages than to functional lang.
I agree completely – it's slightly tangential to your direct question, but I think a more idiomatic (functional) approach would be:
let divideIntoEquivalenceClasses keyf seq =
(System.Collections.Generic.Dictionary(), seq)
||> Seq.fold (fun dict v ->
let key = keyf v
match dict.TryGetValue key with
| false, _ -> dict.Add (key, ResizeArray(Seq.singleton v))
| _, prev -> prev.Add v
dict)
|> Seq.map (function KeyValue (k, v) -> k, Seq.readonly v)
This allows sufficient type inference to obviate the need for your question in the first place.
The workarounds proposed by the other answers are all good. Just to clarify based on your latest updates, let's consider two blocks of code:
let empties = Array.create 100 []
as opposed to:
let empties = Array.create 100 []
empties.[0] <- [1]
In the second case, the compiler can infer that empties : int list [], because we are inserting an int list into the array in the second line, which constrains the element type.
It sounds like you'd like the compiler to infer a generic value empties : 'a list [] in the first case, but this would be unsound. Consider what would happen if the compiler did that and we then entered the following two lines in another batch:
empties.[0] <- [1] // treat 'a list [] as int list []
List.iter (printfn "%s") empties.[0] // treat 'a list [] as string list []
Each of these lines unifies the generic type parameter 'a with a different concrete type (int and string). Either of these unifications is fine in isolation, but they are incompatible with each other and would result in treating the int value 1 inserted by the first line as a string when the second line is executed, which is clearly a violation of type safety.
Contrast this with an empty list, which really is generic:
let empty = []
Then in this case, the compiler does infer empty : 'a list, because it's safe to treat empty as a list of different types in different locations in your code without ever impacting type safety:
let l1 : int list = empty
let l2 : string list = empty
let l3 = 'a' :: empty
In the case where you make empties the return value of a generic function:
let empties() = Array.create 100 []
it is again safe to infer a generic type, since if we try our problematic scenario from before:
empties().[0] <- [1]
List.iter (printfn "%s") (empties().[0])
we are creating a new array on each line, so the types can be different without breaking the type system.
Hopefully this helps explain the reasons behind the limitation a bit more.
I'm writing a quick DB perf test, and chose F# so I can get more practice.
I've created a method, measureSelectTimes, which has the signature Guid list * Guid list -> IDbCommand -> TimeSpan * TimeSpan.
Then, I call it:
let runTests () =
let sqlCeConn : IDbConnection = initSqlCe() :> IDbConnection
let sqlServerConn : IDbConnection = initSqlServer() :> IDbConnection
let dbsToTest = [ sqlCeConn; sqlServerConn ]
let cmds : seq<IDbCommand> = dbsToTest |> Seq.map initSchema
let ids : seq<Guid list * Guid list> = cmds |> Seq.map loadData
let input = Seq.zip ids cmds
let results = input |> Seq.map (fun i -> measureSelectTimes (fst i) (snd i))
// ...
I've annotated explicitly with types to clarify.
What I can't figure out is how to call measureSelectTimes without the lambda. I'd like to partially apply the ids to it like this: ids |> Seq.map measureSelectTimes but then I don't know what to do with the resulting partially applied functions to then map onto the cmds. What's the syntax for this?
You can use Seq.map2:
Seq.map2 measureSelectTimes ids cmds
Or
(ids, cmds) ||> Seq.map2 measureSelectTimes
Your measureSelectTimes function takes two arguments as separate arguments, but you instead need a function that takes them as a tuple. One option is to just change the function to take a tuple (if it is logical for the arguments to be tupled).
Alternative, you can write a cobinator that turns a function taking two arguments into a function taking tuple. This is usually called uncurry and it exists in some functional language:
let uncurry f (a, b) = f a b
Then you can write:
input |> Seq.map (uncurry measureSelectTimes)
This looks okay for a simple use like this, but I think that using combinators too much in F# is not a good idea as it makes code difficult to read for less experienced functional programmers. I would probably write something like this (because I find that more readable):
[ for (time1, time2) in input -> measureSelectTimes time1 time2 ]
One approach is to change the signature of measureSelectTimes to
(Guid list * Guid list) * IDbCommand -> TimeSpan * TimeSpan
Then you can change the map call to
let results = input |> Seq.map measureSelectTimes
// or
let results = Seq.map measureSelectTimes input