I'm currently taking calsses in F#, we have gotten the following assigment https://i.stack.imgur.com/Z4Hmy.png
And I'm currently just focusing on getting isEmpty to work, but it not that easy :D
I have tried to write the following in 'intLinked.fs':
module IntLinkedList
type intLinkedList = Nil | Cons of int * intLinkedList
//Check if a stack is empty
let isEmpty (stck: intLinkedList): bool = stck.IsEmpty
I'm certain that I have written my app.fsproj correct, but when I run the following debug message apears:
error FS0039: The type 'intLinkedList' does not define the field,
constructor or member 'IsEmpty'
The code works when:
I write the following in intLinkedList.fsi:
module IntLinkedList
type stack = int list
val isEmpty : stack -> bool
I write the following in intLinkedList.fs:
module IntLinkedList
type stack = int list
let isEmpty (stck: stack): bool = stck.IsEmpty
I write the following in intLinkedApp.fsx:
open IntLinkedList
let emptyList = []
isEmpty emptyList |> printfn " Empty list is empty : %A"
It's when I try to use the type intLinkedList I run into problems.
As far as I understand Nil is just an empty list [], but it doesn't seem to act like it.
Is it posible that any of you might be able to give me a hint, so I can get back on track?
Thanks in advance
Nil is not "just an empty list". Your type intLinkedList is a completely new type, not related to F# built-in list at all.
It has the same general structure as F# built-in list. In smartypants talk you might say it's "isomorphic" to the F# built-in list. But it's not the same thing.
This means that you can't use methods and functions that work with F# built-in lists on your custom type. You have to implement them yourself.
For example, the IsEmpty property might be implemented something like:
type intLinkedList = Nil | Cons of int * intLinkedList
with
member this.IsEmpty = match this with Nil -> true | Cons _ -> false
Related
I just started my first F# project and coming from the JVM world, I really like Kotlin's nullability syntax and was wondering how I could achieve similarily compact syntax in F#.
Here's an example:
class MyClass {
fun doSomething() {
// ...
}
}
// At some other place in the code:
val myNullableValue: MyClass? = null
myNullableVallue?.doSomething()
What this does:
If myNullableValue is not null, i.e. there is some data, doSomething() is called on that object.
If myNullableValue is null (like in the code above), nothing happens.
As far as I see, the F# equivalent would be:
type MyClass =
member this.doSomething() = ()
type CallingCode() =
let callingCode() =
let myOptionalValue: MyClass option = None
match myOptionalValue with
|Some(x) -> x.doSomething()
|None -> ()
A stamement that is 1 line long in Kotlin is 3 lines long in F#. My question is therefore whether there's a shorter syntax that acomplishes the same thing.
There is no built-in operator for doing this in F# at the moment. I suspect that the reason is that working with undefined values is just less frequent in F#. For example, you would never define a variable, initialize it to null and then have some code that may or may not set it to a value in F#, so the usual way of writing F# eliminates many of the needs for such operator.
You still need to do this sometimes, for example when using option to represent something that can legitimately be missing, but I think this is less frequent than in other languages. You also may need something like this when interacting with .NET, but then it's probably good practice to handle nulls first, before doing anything else.
Aside from pattern matching, you can use Option.map or an F# computation expression (there is no standard one, but it's easy to use a library or define one - see for example). Then you can write:
let myOptionalValue: MyClass option = None
// Option #1: Using the `opt` computation expression
opt { let! v = myOptionalValue
return v.doSomething() }
// Option #2: Using the `Option.map` function
myOptionalValue |> Option.map (fun v -> v.doSomething() )
For reference, my definition of opt is:
type OptionBuilder() =
member x.Bind(v,f) = Option.bind f v
member x.Return v = Some v
member x.ReturnFrom o = o
member x.Zero () = None
let opt = OptionBuilder()
The ?. operator has been suggested to be added to F#.
https://github.com/fsharp/fslang-suggestions/issues/14
Some day it will be added, I hope soon.
I am trying to abstract inserting objects of different types into sql tables of similar structure. Here's what I'm trying to do:
class TableAccess[A : Meta](table: String) {
def insert(key: String, a: A): ConnectionIO[Unit] = {
(fr"insert into " ++ Fragment.const(table) ++ fr" values ($key, $a);").update.run.map(_ => ())
}
}
But I get this compile error:
[error] diverging implicit expansion for type doobie.util.param.Param[A]
[error] starting with method fromMeta in object Param
[error] (fr"insert into " ++ Fragment.const(table) ++ fr" values ($key, $a);").update.run.map(_ => ())
All I can find in the documentation is:
doobie allows you to interpolate values of any type (and options
thereof) with an Meta instance, which includes...
But it seems that is not enough in this case; what's the right typeclass/imports/conversions I need?
I'll go ahead an answer my own question, almost a year later. I never fully understood what was happening, and I have since updated to a newer version of doobie, so I am not sure how relevant this is. But now the documentation contains this clue:
Note: it is important to understand that Meta exists only to introduce
Get/Put pairs into implicit scope. You should never demand Meta as
evidence in user code: instead demand Get, Put, or both.
def foo[A: Meta](...) // don't do this
def foo[A: Get: Put](...) // ok
And indeed, between that change and the new version, this now compiles just fine for me:
class TableAccess[A: Get: Put](table: String) {
When the compiler is resolving implicit its searches for one of a specific type in the current scope. Here it seems like his finding more than one in his tree search.
It's not a matter of a missing typeclass or imports, it's more like you have too many of them and the compiler cant figure the right one.
Try removing some implicit and see how that works or pass them explicitly.
One way I resolved this was to localize the type parameters (and their evidence) onto the method (on a static/companion object), and then it compiled.
Something like
object MinimalGood {
def good[A: Meta, B: Meta](a: A, b: B): Update0 =
sql"""$a $b""".update
}
I want to have a function that determines if a type is a function type, like this:
isFunction : Type -> Bool
isFunction (a -> b) = True
isFunction _ = False
This returns True for all inputs, however. How can I make this work?
Matching on a Type is known as type-casing. Being able to do this would break a concept called parametricity which is very valuable.
https://stackoverflow.com/a/23224110/108359
Idris does not support type-casing. I think it may have tried at one stage, but people gave up on that quickly. It also used to give errors when type-casing was attempted but the check caused bugs and so has been disabled for now.
While you don't get errors any more, you can't provide an implementation of Type -> Bool which will work differently to const True or const False.
So sorry, this function is not possible. It might sound like a limitation but every time I thought I had a motivation for a function similar to this, I eventually figured out that I could have structured my program in a more explicit way instead.
The question here would be: Why do you need something like this?
Do you need to find out if any arbitrary Idris type is a function or not? Or do you just need to find this out from a specific subset of Idris types?
I can't figure out possible applications for the 1st one, but if it's the 2nd one you need, then you could define your own embedded language for dealing with values with these types, like this:
data Ty = TyNat | TyString | TyFun Ty Ty | ...
isFunction : Ty -> Bool
isFunction (TyFun _ _) = True
isFunction _ = False
Check out this talk for more information about how to implement these kind of embedded languages:
https://vimeo.com/61663317
I'm curious to understand why this error happens and which is the best way to get around it.
I have a couple of files types.ml and types.mli which define a variant type value that can be of many different builtin OCaml types (float, int, list, map, set, etc..).
Since I have to use the std-lib over this variant type I needed to concretize the Set module through the functor to be able to use sets of value type by defining the ValueSet module.
The final .ml file is something like:
module rec I :
sig
type value =
Nil
| Int of int
| Float of float
| Complex of Complex.t
| String of string
| List of (value list) ref
| Array of value array
| Map of (value, value) Hashtbl.t
| Set of ValueSet.t ref
| Stack of value Stack.t
...
type t = value
val compare : t -> t -> int
end
= struct
(* same variant type *)
and string_value v =
match v with
(* other cases *)
| Set l -> sprintf "{%s} : set" (ValueSet.fold (fun i v -> v^(string_value i)^" ") !l "")
end
and OrderedValue :
sig
type t = I.value
val compare : t -> t -> int
end
= struct
type t = I.value
let compare = Pervasives.compare
end
and ValueSet : Set.S with type elt = I.value = Set.Make(I)
As you can see I had to define the ValueSet module from the functor to be able to use that datatype. The problem occurs when I want to use that module inside the declaration of I. So that I obtain the following error:
Error: Cannot safely evaluate the definition of the recursively-defined module I
Why does this happen? Which is a good way to solve it? And just to know, is my approach to what I'm trying to do correct? Apart from that it works as intended (I'm able to use the ValueSet type with my operations in other modules, but I have to comment the involved line in types.ml to pass compilation phase).
I tried to remove all the superfluous code and reduce the code to essential needed to investigate this error.. if it's not enought just ask :)
EDIT: according to OCaml reference we have that
Currently, the compiler requires that all dependency cycles between the recursively-defined module identifiers go through at least one “safe” module. A module is “safe” if all value definitions that it contains have function types typexpr1 -> typexpr2.
This is everything I found so far, but I don't get the exact meaning..
Thank in advance
After fixing the obvious errors, your example does compile (with OCaml 3.10, but I think this hasn't changed since recursive modules were introduced in 3.07). Hopefully my explanations below will help you find what, amongst the definitions you left out, caused your code to be rejected.
Here is some example code that is accepted:
module rec Value : sig
type t =
Nil
| Set of ValueSet.t
val compare : t -> t -> int
val nil : t
(*val f_empty : unit -> t*)
end
= struct
type t =
Nil
| Set of ValueSet.t
let compare = Pervasives.compare
let nil = Nil
(*let f_empty () = Set ValueSet.empty*)
end
and ValueSet : Set.S with type elt = Value.t = Set.Make(Value)
At the expression level, the module Value has no dependency on ValueSet. Therefore the compiler generates the code to initialize Value before the code to initialize Value, and all goes well.
Now try commenting out the definition of f_empty.
File "simple.ml", line 11, characters 2-200:
Cannot safely evaluate the definition of the recursively-defined module Value
Now Value does depend on ValueSet, and ValueSet always depends on Value because of the compare function. So they are mutually recursive, and the “safe module” condition must apply.
Currently, the compiler requires that all dependency cycles between the
recursively-defined module identifiers go through at least one "safe" module. A
module is "safe" if all value definitions that it contains have function types
typexpr_1 -> typexpr_2.
Here, ValueSet isn't safe because of ValueSet.empty, and Value isn't safe because of nil.
The reason to the “safe module” condition is the chosen implementation technique for recursive module:
Evaluation of a recursive module definition proceeds
by building initial values for the safe modules involved, binding all
(functional) values to fun _ -> raise Undefined_recursive_module. The defining
module expressions are then evaluated, and the initial values for the safe
modules are replaced by the values thus computed.
If you comment out the declaration of nil in the signature of Value, you can leave the definition and declaration of f_empty. That's because Value is now a safe module: it contains only functions. It's ok to leave the definition of nil in the implementation: the implementation of Value is not a safe module, but Value itself (which is its implementation coerced to a signature) is safe.
I'm really not sure what kind of syntax you're using in the signature that allows let ... I'm going to assume it was a mistake while reducing the code for us. You also don't need that OrderedType definition, possibly another fiddling error for us, since you don't use it in parameterisation of the Set module.
Aside from that, I have no problem running the following in the toplevel. Since this works pretty directly, I am unsure how you're getting that error.
module rec Value :
sig
type t =
| Nil
| Int of int
| Float of float
| String of string
| Set of ValueSet.t
val compare : t -> t -> int
val to_string : t -> string
end = struct
type t =
| Nil
| Int of int
| Float of float
| String of string
| Set of ValueSet.t
let compare = Pervasives.compare
let rec to_string = function
| Nil -> ""
| Int x -> string_of_int x
| Float x -> string_of_float x
| String x -> x
| Set l ->
Printf.sprintf "{%s} : set"
(ValueSet.fold (fun i v -> v^(to_string i)^" ") l "")
end
and ValueSet : Set.S with type elt = Value.t = Set.Make (Value)
I'm quite stuck with the following functor problem in OCaml. I paste some of the code just to let you understand. Basically
I defined these two modules in pctl.ml:
module type ProbPA = sig
include Hashtbl.HashedType
val next: t -> (t * float) list
val print: t -> float -> unit
end
module type M = sig
type s
val set_error: float -> unit
val check: s -> formula -> bool
val check_path: s -> path_formula -> float
val check_suite: s -> suite -> unit
end
and the following functor:
module Make(P: ProbPA): (M with type s = P.t) = struct
type s = P.t
(* implementation *)
end
Then to actually use these modules I defined a new module directly in a file called prism.ml:
type state = value array
type t = state
type value =
| VBOOL of bool
| VINT of int
| VFLOAT of float
| VUNSET
(* all the functions required *)
From a third source (formulas.ml) I used the functor with Prism module:
module PrismPctl = Pctl.Make(Prism)
open PrismPctl
And finally from main.ml
open Formulas.PrismPctl
(* code to prepare the object *)
PrismPctl.check_suite s.sys_state suite (* error here *)
and compiles gives the following error
Error: This expression has type Prism.state = Prism.value array
but an expression was expected of type Formulas.PrismPctl.s
From what I can understand there a sort of bad aliasing of the names, they are the same (since value array is the type defined as t and it's used M with type s = P.t in the functor) but the type checker doesn't consider them the same.
I really don't understand where is the problem, can anyone help me?
Thanks in advance
(You post non-compilable code. That's a bad idea because it may make it harder for people to help you, and because reducing your problem down to a simple example is sometimes enough to solve it. But I think I see your difficulty anyway.)
Inside formulas.ml, Ocaml can see that PrismPctl.s = Pctl.Make(Prism).t = Prism.t; the first equality is from the definition of PrismPctl, and the second equality is from the signature of Pctl.Make (specifically the with type s = P.t bit).
If you don't write an mli file for Formulas, your code should compile. So the problem must be that the .mli file you wrote doesn't mention the right equality. You don't show your .mli files (you should, they're part of the problem), but presumably you wrote
module PrismPctl : Pctl.M
That's not enough: when the compiler compiles main.ml, it won't know anything about PrismPctl that's not specified in formulas.mli. You need to specify either
module PrismPctl : Pctl.M with type s = Prism.t
or, assuming you included with type s = P.t in the signature of Make in pctl.mli
module PrismPctl : Pctl.M with type s = Pctl.Make(Prism).s
This is a problem I ran into as well when learning more about these. When you create the functor you expose the signature of the functor, in this case M. It contains an abstract type s, parameterized by the functor, and anything more specific is not exposed to the outside. Thus, accessing any record element of s (as in sys_state) will result in a type error, as you've encountered.
The rest looks alright. It is definitely hard to get into using functors properly, but remember that you can only manipulate instances of the type parameterized by the functor through the interface/signature being exposed by the functor.