Arrow should only appear in cases expressions and anonymous functions

Question

I am trying to check if a given key exist in a dict given the key. I am relatively new to Elm and functional programming, so I am not sure where I am going wrong.

The error I am receiving is:

An arrow should only appear in cases expressions and anonymous functions. Maybe you want > or >= instead?

Here's my attempt at returning true or false

dictExist : comparable -> Dict comparable v -> Bool
dictExist dict key =
    Dict.get key dict        
            Just -> True 
            Maybe.Maybe -> False

On another note I have tried the Dict.member as well, but no success either and would assume instead of the Dict.get I should be using Dict.member for this...

Answer 1

There are four problems with your code:

1. As the error points out, you're using the arrow outside a case ... of expression.
2. The Maybe type's Just constructor has an accompanying value, the item from the dict, but you don't bind it to anything. You have to explicitly discard it by assigning it to the wildcard pattern, _.
3. Maybe.Maybe is not a constructor. This should be Nothing, which is the other constructor of the Maybe type.
4. You've flipped the argument order of dictExist

Having fixed these problems, this code should work:

dictExist : comparable -> Dict comparable v -> Bool
dictExist key dict =
    case Dict.get key dict of        
        Just _ -> True 
        Nothing -> False

But this is really just a reimplmentation of Dict.member, which has the exact same type signature. Replacing any usage of dictExist with Dict.member should therefore work exactly the same.