CODEWITHSUNDEEP
typesocaml
Ecila
Ecila

Reputation: 57

type error in ocaml - This expression has type (string * 'a) list, but is used with type exp

I, i have this simple program in ocaml 

type tree = Node of string * exp * tree * tree | Empty 

and exp = Etree of tree |  EInt of int  | List of (string*exp);;

let rec evalNode.......



and rec eval (e:exp) =
(match e with
     | EInt _ -> e
     | Etree(tree) -> (match tree with 
         | Empty -> [] 
         | Node(s,i,t1,t2) -> (evalNode (tree)) )

  )

evalNode : tree -> (string * exp) list = fun

eval : exp -> exp = fun

but return this error but i don't undestand what does means, there aren't much information of ocaml in the web:

This expression has type (string * 'a) list, but is used with type exp.

Etree has a type Node inside, so is type tree, so i can call evalNode with this value because it want a tree type

evalNode return a list of (string*exp) but is a exp so eval can return this

Upvotes: 1

Views: 1218

Answers (2)

Keith Irwin
Keith Irwin

Reputation: 5668

Any expression in Ocaml has to return a single type. So, for example, you can't write something like if a>b then "a" else 7 because it doesn't know whether the type of that expression is string or int.

In your eval function, without meaning to, you're doing something similar. When e is an EInt the first matching option is triggered and it returns e which has type exp. When e is an Etree(Empty) it triggers the second matching option which in turn calls the second match statement and the first matching option within that is triggered, returning [], which is a list having type 'a list (meaning list of unknown type). When e is an Etree(Node(s,i,t1,t2)) the second matching option in the second match statement is triggered, returning evalnode(tree) which we know from what you've written to have type (string * exp) list.

So, the inner match, by itself is fine since it can unify a generic list with a specific list. But combining the outer match is problematic since the first case returns an exp and the second case returns a (string * exp) list so it doesn't know what type the expression should have. I'm not sure why the error says (string * 'a) list rather than (string * exp) list but I suspect you can fix things without having to work that out.

I do also want to note that you have a constructor List of (string*exp) whose subtype is not a list. You may wish this to be List of (string*exp) list and then to have your inner match return List([]) and List(evalNode(tree)) but that's just a guess.

Upvotes: 1

octachron
octachron

Reputation: 18902

evalNode return a list of (string*exp) but is a exp

This is your error: a list of string * exp is not an exp; the compiler cannot guess that you meant to apply the constructor List to build an exp from this list (if the right definition of List happens to be List of (string * exp) list).

Upvotes: 0

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