Compiler: Exposed imported variables can be shadowed

[lydell]

Quick Summary:
Elm does not allow shadowing: https://github.com/elm/compiler/blob/master/hints/shadowing.md
But is is possible to shadow exposed imported variables.

SSCCE

module Main exposing (..)

import List exposing (product)
-- or:
-- import List exposing (..)


product =
    { name = "Toothbrush"
    , price = 2
    }

The above does not produce any compile (shadowing) errors!

• Elm: 0.19.1
• Browser: n/a
• Operating System: macOS Mojave

Additional Details

I think the ideas outlined in https://github.com/elm/compiler/blob/master/hints/shadowing.md applies to shadowing imported variables too. Let’s make a similar example to the name example from that document.

Let’s say were working on _AwesomeDental’s_ website:

import Company exposing (name)
import Html


view =
    Html.h1 [] [ Html.text ("Welcome to " ++ name ++ "’s website!") ]

Some time later, hundreds of lines further down, someone is making a couple of view functions for the company’s new product, the _SuperToothbrush._ The programmer has mentioned the string "SuperToothbrush" over and over, but now their boss says it’s very important to have a ™ sign after the name. Since finding and updating all the occurances was a bit annoying, the programmer extracts a variable so that a future rename would be easier:

name =
    "SuperToothbrush™"

Oops! Now the website heading will say “Welcome to SuperToothbrush™’s website” rather than “Welcome to AwesomeDental’s website!”

[lydell]

Consideration: If this change is made, this code wouldn’t compile anymore:

someResult |> Result.mapError (\e -> "Error: " ++ e)

Because the e would conflict with Basics.e. But I guess one-letter variable names aren’t that good anyway? I’d rename to err or error or errorMessage in the above example.

Oh, and I should mention the example from the Slack conversation as well. This is forbidden (conflict with Basics.round):

import Round exposing (round)

But this is OK:

import Round

round = Round.round

Which is hard to explain to a beginner.

[lydell]

Another consideration: If this change is made, then the Round package wouldn’t be able to provide the Round.round function anymore, because it wouldn’t be able to create it in the first place (conflicts with Basics.round). Hmm 🤔

[rlefevre]

This is on purpose, see https://github.com/elm/compiler/issues/1806.

[lydell]

Thanks!

I might still be worth disallowing shadowing when an explicit exposing list is used? I mean, if I have bothered typing exposing (length) then why would I ever want to do length x = ...? In other words, only allow shadowing of variables coming from exposing (..).

[pravdomil]

This is on purpose, see #1806.

https://github.com/elm/compiler/blob/master/hints/shadowing.md should mention that