In the following method, I get the following error: error FS0001: This expression was expected to have type bool but here has type Outcome list * ProbTree -> bool when trying to call isSample in the if expression. I have no clue why I get the message.
isSample(os, t) is currently of type 'a * 'b -> Outcome list * ProbTree -> bool. Any ideas? What I don't understand is why it expects type bool all of a sudden.
let rec isSample(os, t) = function
| ([], Branch(_, p, b1, b2)) -> false
| (os, Branch(_, p, b1, b2)) when List.length os = 1 -> true
| (h::t, Branch(_, p, b1, b2)) ->
if h = S then isSample(t, b1) else isSample(t, b2) //This is where I get the error
| _ -> false
type Outcome = | S | F
type Sample = Outcome list
type ProbTree = | Branch of string * float * ProbTree * ProbTree
| Leaf of string
Related
Let say I have DU of three types and a function which accepts this DU as a parameter:
type A = decimal<p>
type B = decimal<p>
type C = decimal<p>
type ABC = A of A | B of B | C of C
let myfunc (val: ABC) =
match val with
| A v -> ...
| B v -> ...
| C v -> ...
Is there a better way to define a function which accepts DU of only A and B without defining new type explicity?
This is a working example:
type AB = A2 of A | B2 of B
let myfunc2 (val: AB) =
match val with
| A2 v -> ...
| B2 v -> ...
This is how I would like it to be:
let myfunc2 (val: A|B) =
match val with
| A v -> ...
| B v -> ...
The language feature you're asking for is open variants, which exists in OCaml, but not in F#. For instance,
type ABC = A of int | B of int | C of int
type AB = A of int | B of int
let myfunc (x: AB) = ...
is legal in OCaml. This can also be achieved with type-erased DUs (vote for it here!).
DU cases in F# are not actual types, but the base type (ABC) with a tag denoting which union case it is. And since there are no types which exist separately, it follows that they cannot be used as a type constraint.
You can however try and work around this with Choice. The idea is to build single case DUs and then combine them with Choice<T1, T2...>.
type A = A of int
type B = B of int
type C = C of int
type ABC = Choice<A, B, C>
type AB = Choice<A, B>
let matchAb (x: AB) =
match x with
| Choice1Of2 (A a) -> a
| Choice2Of2 (B b) -> b
In F# I am trying to get last element of give list. I wrote below code
let rec findLast t =
match t with
| hd :: [] -> hd
| hd :: tl -> findLast tl
| _ -> -1
printfn "%A" (findLast [1,2,3,4,5])
But when I tried to execute it in F# Interactive it complain as below
error FS0001: This expression was expected to have type
int but here has type
'a * 'b * 'c * 'd * 'e
I just want to know what is wrong in above code. I know there are different smart and elegant ways to get last element from list in F#. But I am interested to know what is wrong in above code ?
1,2,3,4,5 is a tuple. 'a * 'b * 'c * 'd * 'e is a tuple definition. Create a list with semicolons [1;2;3;4;5]. [1,2,3,4,5] is a list of tuples with one item which is a quintuple.
let rec findLast t =
match t with
| hd :: [] -> hd
| hd :: tl -> findLast tl
| _ -> -1
printfn "%A" (findLast [1;2;3;4;5])
Try this one:
let rec lastElem = function
| [] -> None
| [x] -> Some x
| x::xs -> lastElem xs
You can try it in the REPL:
> lastElem [1;2;3];;
val it : int option = Some 3
> lastElem ["a";"b";"c"];;
val it : string option = Some "c"
As #Phillip-Scott-Givens pointed out, you've likely made a totally common (especially for C#'ers), error and used a comma to separate a list instead of a semi-colon.
This results in a tuple list [(1, 2, 3, 4, 5)] and not an integer list [1;2;3;4;5]. Getting unexpected asterisks in your type definitions is a symptom of this :)
That said, here a few different functions that get the last value from your tuple, list, and tuple list (ref: https://stackoverflow.com/a/1175123/5470873):
// Data:
let tuples = [ (1,2,3,4,5); ] // = [1,2,3,4,5]
let firstListElement = tuples.[0]
// Access:
let rec lastItemInList = function
| hd :: [] -> hd
| hd :: tl -> lastItemInList tl
| _ -> failwith "Empty list."
let lastValueOfFirstItem = function
| (_, _, _, _, last) :: _ -> last
| _ -> -1
let lastValueOfTuple = function _, _, _, _, last -> last
// same as: let lastValueOfTuple myTuple =
// match myTuple with
// | (_, _, _, _, last) -> last
// Examples:
tuples |> lastItemInList // val it : int * int * int * int * int = (1, 2, 3, 4, 5)
tuples |> lastValueOfFirstItem // val it : int = 5
tuples |> List.map lastValueOfTuple // val it : int list = [5]
firstListElement |> lastValueOfTuple // val it : int = 5
I have the following function for parsing an integer:
let rec digits = function
| head::tail when System.Char.IsDigit(head) ->
let result = digits tail
(head::(fst result), snd result)
| rest -> ([], rest)
If I change this function to be an active recognizer, it no longer compiles.
let rec (|Digits|) = function
| head::tail when System.Char.IsDigit(head) ->
let result = Digits tail
(head::(fst result), snd result)
// ^^^^^^ ^^^^^^ see error*
| rest -> ([], rest)
*error FS0001: This expression was expected to have type char list * 'a but here has type
char list
let rec (|Digits|) = function
| head::(Digits (a, b)) when System.Char.IsDigit(head) -> (head::a, b)
| rest -> ([], rest)
NOTE:
if you want to use active pattern as a function you still can do it:
let rec (|Digits|) = function
| head::tail when System.Char.IsDigit(head) ->
let a, b = (|Digits|) tail
(head::a, b)
| rest -> ([], rest)
I am writing a simple expressions parser in F# and for each operator I want only to support a certain number of operands (e.g. two for Modulo, three for If). Here is what I have:
type Operator =
| Modulo
| Equals
| If
let processOperator operands operator =
match operator with
| Modulo ->
match operands with
| [ a:string; b:string ] -> (Convert.ToInt32(a) % Convert.ToInt32(b)).ToString()
| _ -> failwith "wrong number of operands"
| Equals ->
match operands with
| [ a; b ] -> (a = b).ToString()
| _ -> failwith "wrong operands"
| If ->
match operands with
| [ a; b; c ] -> (if Convert.ToBoolean(a) then b else c).ToString()
| _ -> failwith "wrong operands"
I would like to get rid of or simplify the inner list matches. What is the best way to accomplish this? Should I use multiple guards ?
open System
type Operator =
| Modulo
| Equals
| If
let processOperator operands operator =
match (operator, operands) with
| Modulo, [a: string; b] -> string ((int a) % (int b))
| Equals, [a; b] -> string (a = b)
| If, [a; b; c] -> if Convert.ToBoolean(a) then b else c
| _ -> failwith "wrong number of operands"
But I would suggest to move this logic of the operands to the parser, this way you get a clean operator expression, which is more idiomatic and straight forward to process, at the end you'll have something like this:
open System
type Operator =
| Modulo of int * int
| Equals of int * int
| If of bool * string * string
let processOperator = function
| Modulo (a, b) -> string (a % b)
| Equals (a, b) -> string (a = b)
| If (a, b, c) -> if a then b else c
Fold in the operands matching:
let processOperator operands operator =
match operator, operands with
| Modulo, [a; b] -> (Convert.ToInt32(a) % Convert.ToInt32(b)).ToString()
| Equals, [a; b] -> (a = b).ToString()
| If, [ a; b; c ] -> (if Convert.ToBoolean(a) then b else c).ToString()
| _ -> failwith "wrong number of operands"
Better yet, if you can, change the datatype to the following.
type Operator =
| Modulo of string * string
| Equals of string * string
| If of string * string * string
Then in the match, you can no longer fail.
Can someone explain why compiler is giving me this error
Type mismatch. Expecting a
'a [] -> string
but given a
'a [] -> 'a []
The type 'string' does not match the type ''a []'
on this code snippet:
let rotate s: string =
[|for c in s -> c|]
|> Array.permute (function | 0 -> (s.Length-1) | i -> i-1)
while the one below compiles just fine:
let s = "string"
[|for c in s -> c|]
|> Array.permute (function | 0 -> (s.Length-1) | i -> i-1)
Your first snippet defines function rotate with return type of string.
Try to change it to:
let rotate (s: string) =
[|for c in s -> c|]
|> Array.permute (function | 0 -> (s.Length-1) | i -> i-1)
In this form you define a function with one string argument (I suppose that's what you wanted) and inferred return type.