Documented declConflict errors

[Woody88]

I am not sure if there are other cases when this error occurs, but I think that this should be fine? I am pretty sure that there is a better way to explain this, but this is what I came up with quickly.

[hdgarrood]

I think this is a little overly specific, as there are indeed a few other cases where this can show up. For example, repeated data constructors, or data constructors whose names conflict with type class names. See tests/purs/failing/DeclConflict*.purs in the compiler repo for more examples.

I don't think your suggested fix really works, though, because writing, say, data X = Y is very different from writing data X = X Y. The former is a type with precisely one inhabitant which you've called Y, the latter is a type which has the same number of inhabitants as some other existing type Y. I think the suggested fix should be to either come up with new names which don't conflict, or to put the conflicting names in a separate module.

[hdgarrood]

Just to be clear, this file doesn't have to exhaustively list the possibilities for an error like this to occur; I think that would probably end up being counterproductive. I think we should just provide a couple of very basic examples, around the minimum size required to produce this error.

[thomashoneyman]

@Woody88 what do you think of an update to your PR to something like this?

---

DeclConflict Error

Example

Each of these pairs will fail with a DeclConflict error.

module Main where

data T = Fail | Fail

data T1 = Fail1
data T2 = Fail1

class Fail2
data T3 = Fail2

Cause

This error occurs when a data constructor, type, or type class conflicts with another data constructor, type, or type class.

Fix

This can be fixed by using non-conflicting names or by putting conflicting names into a separate module. Then, if the names are later used together in a module they can be distinguished via a qualified import:

module Main where

import Module1 as Module1 -- includes: data Works = Works
import Module2 as Module2 -- includes: data Works = Works

x :: Module1.Works
x = Module1.Works

y :: Module2.Works
y = Module2.Works

[thomashoneyman]

I agree with @hdgarrood that this needs some changes before it's updated.

[Woody88]

Hi @thomashoneyman @hdgarrood!

No problem, I will add the proposed changes tomorrow.

[hdgarrood]

Thanks!