I'm trying to make a function that extends a symbolTable with a list of Decl
Definitions:
type Typ = |Integer
|Boolean
|Ft of Typ list * Typ;;
type Decl = string * Typ;;
type symbolTable = Map<string, Typ>;;
I'm trying to do it the following way:
let extendST (st: symbolTable) (lst: Decl list) : symbolTable =
let mutable returnValue = st
for (x,y) in lst do
returnValue = returnValue.Add(x,y)
returnValue;;
But apparently nothing is being added to the returnValue (the function returns the same symbolTable as is being input).
I'm obviously new at F# but this is giving me a headache so I hope someone can help me out.
In F#, assignment (to change the value of a mutable variable) is written using <- rather than using = (which means just equality testing). Your code is comparing the two values and then ignoring the result (which is valid, but not what you wanted). The corrected version looks like this:
let extendST (st: symbolTable) (lst: Decl list) : symbolTable =
let mutable returnValue = st
for (x,y) in lst do
returnValue <- returnValue.Add(x,y)
returnValue
However, if you are looking for a functional solution, you can write this using fold:
let extendST (st: symbolTable) (lst: Decl list) : symbolTable =
lst |> List.fold (fun map (k, v) -> Map.add k v map) st
The fold function takes some state (your symbol table) and calculates a new state for each element of a given list lst. Here, the new state is a new map extended with the key-value pair k,v.
Related
I'm currently learning F# and hitting a few stumbling blocks; I think a lot of it is learning to think functionally.
One of the things I'm learning at the moment are computation expressions, and I want to be able to define a computation expression that handles some tracking state, e.g:
let myOptions = optionListBuilder {
let! opt1 = {name="a";value=10}
let! opt2 = {name="b";value=12}
}
I want to be able to have it so that myOptions is a Option<'T> list, so each let! bind operation effectively causes the builder to "track" the defined options as it goes along.
I don't want to have to do it using mutable state - e.g. having a list maintained by the builder and updated with each bind call.
Is there some way of having it so that this is possible?
Update: The resultant Option<'T> list type is just representative, in reality I'll likely have an OptionGroup<'T> type to contain a list as well as some additional information - so as Daniel mentioned below, I could use a list comprehension for a simple list.
I wrote a string builder computation expression here.
open System.Text
type StringBuilderUnion =
| Builder of StringBuilder
| StringItem of string
let build sb =
sb.ToString()
type StringBuilderCE () =
member __.Yield (txt : string) = StringItem(txt)
member __.Yield (c : char) = StringItem(c.ToString())
member __.Combine(f,g) = Builder(match f,g with
| Builder(F), Builder(G) ->F.Append(G.ToString())
| Builder(F), StringItem(G)->F.Append(G)
| StringItem(F),Builder(G) ->G.Append(F)
| StringItem(F),StringItem(G)->StringBuilder(F).Append(G))
member __.Delay f = f()
member __.Zero () = StringItem("")
member __.For (xs : 'a seq, f : 'a -> StringBuilderUnion) =
let sb = StringBuilder()
for item in xs do
match f item with
| StringItem(s)-> sb.Append(s)|>ignore
| Builder(b)-> sb.Append(b.ToString())|>ignore
Builder(sb)
let builder1 = new StringBuilderCE ()
Noticed the underlying type is immutable (the contained StringBuilder is mutable, but it doesn't have to be). Instead of updating the existing data, each yield combines the current state and the incoming input resulting in a new instance of StringBuilderUnion You could do this with an F# list since adding an element to the head of the list is merely the construction of a new value rather than mutating the existing values.
Using the StringBuilderCE looks like this:
//Create a function which builds a string from an list of bytes
let bytes2hex (bytes : byte []) =
string {
for byte in bytes -> sprintf "%02x" byte
} |> build
//builds a string from four strings
string {
yield "one"
yield "two"
yield "three"
yield "four"
} |> build
Noticed the yield instead of let! since I don't actually want to use the value inside the computation expression.
SOLUTION
With the base-line StringBuilder CE builder provided by mydogisbox, I was able to produce the following solution that works a charm:
type Option<'T> = {Name:string;Item:'T}
type OptionBuilderUnion<'T> =
| OptionItems of Option<'T> list
| OptionItem of Option<'T>
type OptionBuilder () =
member this.Yield (opt: Option<'t>) = OptionItem(opt)
member this.Yield (tup: string * 't) = OptionItem({Name=fst tup;Item=snd tup})
member this.Combine (f,g) =
OptionItems(
match f,g with
| OptionItem(F), OptionItem(G) -> [F;G]
| OptionItems(F), OptionItem(G) -> G :: F
| OptionItem(F), OptionItems(G) -> F :: G
| OptionItems(F), OptionItems(G) -> F # G
)
member this.Delay f = f()
member this.Run (f) = match f with |OptionItems items -> items |OptionItem item -> [item]
let options = OptionBuilder()
let opts = options {
yield ("a",12)
yield ("b",10)
yield {Name = "k"; Item = 20}
}
opts |> Dump
F# supports list comprehensions out-of-the-box.
let myOptions =
[
yield computeOptionValue()
yield computeOptionValue()
]
I am trying to iterate through an IDictionary (reasons explained later...) in F#, and round each value to a specified precision. Essentially this is what I'm trying to do:
List.iter (fun(x) -> a.Item(x) <- Math.Round(a.Item(x), input.precision)) (ICollectionToDoubleList a.Keys)
(where ICollectionToDoubleList takes the ICollection a.Keys and casts it to a double list).
However since you can't alter mutable variables inside closures, this doesn't compile.
My first attempt at a solution was this:
List.iter (fun(x) -> let p = Math.Round(a.Item(x), input.precision)
a.Item(x) := p
) (ICollectionToDoubleList a.Keys)
However I'm getting the error:
This expression was expected to have type
'a ref
but here has type
double
on a.Item(x)
I could convert the IDictionary into two lists (or a list of tuples), perform the rounding, and re-cast into an IDictionary, but this seems a bit messy and convoluted.
Any guidance greatly appreciated.
EDIT:
I forgot to mention a was defined as:
let mutable (a : IDictionary<double,double>) = ...
I think you want
a.Item(x) <- p
In F# you use <- to assign to mutable values, whilst := assign to ref values.
You could even use
a.[x] <- p
for a slightly simpler version.
Explaination of what mutable means (it behaves like the opposite of const in C)
let mutable m = [|1|]
let t = [|1|]
m.[0] <- 0
t.[0] <- 0 //neither of these change m or t - only elements so they are fine
m <- [|1;2;3;|] //fine as m is mutable
t <- [|1;2;3;|] //not allowed as t is not mutable
If you are used to const in C, the above are roughly equivalent to
int* m = {1};
const int* t = {1}
note, neither is equivalent to
const int* q const = {1}
which is I think what you thought not mutable meant.
Ok so I've discovered the answer...
I have defined a as:
let mutable (a : IDictionary<double,double>) = ...
If I change this to
let (a : IDictionary<double,double>) = ...
then this compiles. It seems a little counter-intuative to me that a non-mutable value can be mutated, but a mutatable variable cannot!!
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.
Is there a way to have mutable function arguments in F#, that would allow something like
let mutable i = 9
let somefun n = n <- 12; ()
somefun i
(* *not* a real-world example *)
I do understand that this can be made to work by wrapping it into a record type
type SomeRec = { mutable i: int }
let ri = { i = 9 }
let someotherfun r = r.i <- 12; ()
and that this can be done in a similar fashion for class members. However, even after browsing through the whole F# Language Specification (yes, I did!), there seems to be no syntax to allow the first case, and the compiler appears to be quite unhappy about my trying this. I was hoping there would be some sort of type annotation, but mutable cannot be used in such.
I also know that I should not be doing this sort of thing in the first place, but the first case (int binding) and the second (record type) are semantically identical, and any such objection would hold for both cases equally.
So I think that I am missing something here.
You can use ref as arguments
let v = ref 0
let mutate r =
r := 100
mutate v
printfn "%d" !v
Or byref keyword
let mutable v = 0
let mutate (r : byref<_>) =
r <- 100
mutate &v
printfn "%d" v
Use byref keyword which is equivalent to C# ref.
See Passing by reference.
Is it just me, or does F# not cater for cyclic lists?
I looked at the FSharpList<T> class via reflector, and noticed, that neither the 'structural equals' or the length methods check for cycles. I can only guess if 2 such primitive functions does not check, that most list functions would not do this either.
If cyclic lists are not supported, why is that?
Thanks
PS: Am I even looking at the right list class?
There are many different lists/collection types in F#.
F# list type. As Chris said, you cannot initialize a recursive value of this type, because the type is not lazy and not mutable (Immutability means that you have to create it at once and the fact that it's not lazy means that you can't use F# recursive values using let rec). As ssp said, you could use Reflection to hack it, but that's probably a case that we don't want to discuss.
Another type is seq (which is actually IEnumerable) or the LazyList type from PowerPack. These are lazy, so you can use let rec to create a cyclic value. However, (as far as I know) none of the functions working with them take cyclic lists into account - if you create a cyclic list, it simply means that you're creating an infinite list, so the result of (e.g.) map will be a potentially infinite list.
Here is an example for LazyList type:
#r "FSharp.PowerPack.dll"
// Valid use of value recursion
let rec ones = LazyList.consDelayed 1 (fun () -> ones)
Seq.take 5 l // Gives [1; 1; 1; 1; 1]
The question is what data types can you define yourself. Chris shows a mutable list and if you write operations that modify it, they will affect the entire list (if you interpret it as an infinite data structure).
You can also define a lazy (potentionally cyclic) data type and implement operations that handle cycles, so when you create a cyclic list and project it into another list, it will create cyclic list as a result (and not a potentionally infinite data structure).
The type declaration may look like this (I'm using object type, so that we can use reference equality when checking for cycles):
type CyclicListValue<'a> =
Nil | Cons of 'a * Lazy<CyclicList<'a>>
and CyclicList<'a>(value:CyclicListValue<'a>) =
member x.Value = value
The following map function handles cycles - if you give it a cyclic list, it will return a newly created list with the same cyclic structure:
let map f (cl:CyclicList<_>) =
// 'start' is the first element of the list (used for cycle checking)
// 'l' is the list we're processing
// 'lazyRes' is a function that returns the first cell of the resulting list
// (which is not available on the first call, but can be accessed
// later, because the list is constructed lazily)
let rec mapAux start (l:CyclicList<_>) lazyRes =
match l.Value with
| Nil -> new CyclicList<_>(Nil)
| Cons(v, rest) when rest.Value = start -> lazyRes()
| Cons(v, rest) ->
let value = Cons(f v, lazy mapAux start rest.Value lazyRes)
new CyclicList<_>(value)
let rec res = mapAux cl cl (fun () -> res)
res
The F# list type is essentially a linked list, where each node has a 'next'. This in theory would allow you to create cycles. However, F# lists are immutable. So you could never 'make' this cycle by mutation, you would have to do it at construction time. (Since you couldn't update the last node to loop around to the front.)
You could write this to do it, however the compiler specifically prevents it:
let rec x = 1 :: 2 :: 3 :: x;;
let rec x = 1 :: 2 :: 3 :: x;;
------------------------^^
stdin(1,25): error FS0260: Recursive values cannot appear directly as a construction of the type 'List`1' within a recursive binding. This feature has been removed from the F# language. Consider using a record instead.
If you do want to create a cycle, you could do the following:
> type CustomListNode = { Value : int; mutable Next : CustomListNode option };;
type CustomListNode =
{Value: int;
mutable Next: CustomListNode option;}
> let head = { Value = 1; Next = None };;
val head : CustomListNode = {Value = 1;
Next = null;}
> let head2 = { Value = 2; Next = Some(head) } ;;
val head2 : CustomListNode = {Value = 2;
Next = Some {Value = 1;
Next = null;};}
> head.Next <- Some(head2);;
val it : unit = ()
> head;;
val it : CustomListNode = {Value = 1;
Next = Some {Value = 2;
Next = Some ...;};}
The answer is same for all languages with tail-call optimization support and first-class functions (function types) support: it's so easy to emulate cyclic structures.
let rec x = seq { yield 1; yield! x};;
It's simplest way to emulate that structure by using laziness of seq.
Of course you can hack list representation as described here.
As was said before, your problem here is that the list type is immutable, and for a list to be cyclic you'd have to have it stick itself into its last element, so that doesn't work. You can use sequences, of course.
If you have an existing list and want to create an infinite sequence on top of it that cycles through the list's elements, here's how you could do it:
let round_robin lst =
let rec inner_rr l =
seq {
match l with
| [] ->
yield! inner_rr lst
| h::t ->
yield h
yield! inner_rr t
}
if lst.IsEmpty then Seq.empty else inner_rr []
let listcycler_sequence = round_robin [1;2;3;4;5;6]