Type error expression of type ?Entry cannot produce

Hey everyone! I am currently watching the Hackin’ on The Internet Computer | ep.3 and I am writing the exact same code.
When I “dfx build” it throws an error that has to do with this part of my code:

public query func get(id0: Nat): async Entry {
func isEq(entry: Entry): Bool {
entry.id == id0;
Array.find(entries, isEq); //Here is the error’s line.

Error: Type error expression of type ?Entry cannot produce.
Can you suggest a solution?

Hi @ThanasisNta! Thanks for reaching out on this. More likely than not, this is probably due to some incompatibility issues with newer versions of the SDK and Replica, as @hansl built the Journey app using a very old version of the SDK - 0.5.5 it looks like. I can’t speak to this specific bug, but it would probably require overhauling the app to work with the current version of the SDK.


Hi Thanasis

The Array.find() function returns an optional Entry, not the Entry itself, so it doesn’t match your public method’s return type. (The Motoko base library has changed a bit since that video was posted).

You could either return an optional Entry from your public method, as async ?Entry , or unwrap the optional value using a switch statement, handling the null case alongside (if you need any pointers on this just let me know and I’ll provide an example).

Also, if you want to take a look at any of the base library modules for your latest installed version you can find them in the $(dfx cache show)/base directory—the comments and function signatures there are a great source of documentation.


Hey @Ori ! I tried to return an optional Entry from my public method as async ?Entry and now I have this error here:

Fatal error: exception Codegen.Compile.CodegenError("Local actors not supported by backend")
Raised at file "codegen/compile.ml", line 47, characters 42-64
Called from file "codegen/compile.ml", line 7117, characters 4-49
Called from file "codegen/compile.ml", line 7153, characters 20-40

I think that might be unrelated to this part of your code?

If you’re using VSCode the Motoko extension can highlight compile errors in the editor.

1 Like

I found the solution for the second error: Multiple Actors Tutorial - type error

Thank you for your help!!!