[SR-6315] Error: Cannot convert value of type to expected argument type for Self Reference Protocol

[swift-ci]

Previous ID	SR-6315
Radar	None
Original Reporter	davideme (JIRA User)
Type	Bug
Status	Reopened
Resolution

Attachment: Download

Environment

Version 9.1 (9B55)

Toolchain: Swift 4.1 Snapshot 2017-11-05 (a)

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, DiagnosticsQoI, TypeChecker
Assignee	@xedin
Priority	Medium

md5: 861161fd3c5321d8b1876268e8c52866

Issue Description:

With the version 4.1 of the compiler, I implemented protocols that use self reference associated type.

I got the following errors:

Cannot convert value of type '(Nuke) -> AttackMonad' to expected argument type '(Nuke) -> _'
Cannot convert value of type '(Target) -> (Nuke, Target)' to expected argument type '(Target) -> _'

Were the compiler is not able to match the types of parameters of 2 methods declared in a protocol.

You can find the code in the following repo:

https://github.com/davideme/Functional-Swift-Error-Handling

In particular the 2 following lines:

• https://github.com/davideme/Functional-Swift-Error-Handling/blob/master/Sources/main.swift#L175

• https://github.com/davideme/Functional-Swift-Error-Handling/blob/master/Sources/main.swift#L179

[belkadan]

This seems like correct behavior to me. You don't know from the constraints you have that NukeMonad's associated type F is an AttackMonad. I think you should be making the generic parameter of map and flatMap be the monad rather than the monad's element—if it's appropriate to use those names for something that changes from one monad to another in the first place.

[swift-ci]

Comment by Davide Mendolia (JIRA)

@belkadan the compiler is capable to autocomplete and identify the it's expecting a Monad as return value. And AttackMonad is defined as an implementation of Monad.

Now, if this shouldn't be possible in Swift, at least the following error messages should be fixed.

About the semantic of this example, we could rename Monad to Container, it's not really the goal of implementing a Monad to play around with what composition allow in swift.

[belkadan]

The code completion should probably say NukeMonad.F but I'm not sure if it has that much information. The error message part is sensible, though.

cc @xedin