Broken redefinition of variable inside if-then expression

[Akirathan]

Inside then branch of an if expression, a variable assignment is syntactically correct. The following snippet should create a new variable named a inside the then expression, and return Nothing.

a = 5
if False then a = 10 else a = 15

But it fails with

Execution finished with an error: Method `=` of 5 (Integer) could not be found.

[JaroslavTulach]

Looks like the situation is more complex. Looking for help by language and parser experts, CCing @wdanilo and @kazcw. Following program offers "block" and "one liner" version of the same code:

block =
    a = 5
    if a == 3 then a = 10 else
        a = 15
        a # needn't be here, still OK without this line

line =
    a = 5
    if a == 3 then a = 10 else a = 15

Running block is fine. Running line generates an error as described in the issue description.

Of course using a = 10 in the "one liner" makes little sense. However using a = 10 in the block (without a being used further) makes little sense as well.

How shall the engine behave? Should the "one liner" behave just like the "block" (without a being returned)? Thank you in advance for your advises.

[JaroslavTulach]

Wojciech wrote:

these 2 codes should behave the same way

I will modify the code so the "one liner" behaves the same as the block version.

[JaroslavTulach]

The Tree we get from the Rust parser is causing the difference. While in the case of "oneliner" it is:

OprApp[" ", "a = 15", Ident["", "a", Ident["", "a", false, 0, false, false]], 
Either{right=Operator[" ", "=", ]}, Number[" ", "15", null, Digits["", "15", null], null]]

in case of the "block" version it is:

Assignment["        ", "a = 15", Ident["", "a", Ident["", "a", false, 0, false, false]], 
Operator[" ", "=", ], Number[" ", "15", null, Digits["", "15", null], null]]

I believe the right fix is to modify the Rust parser to treat even the "oneliner" as an Assignment. Passing to @kazcw

[enso-bot]

Jaroslav Tulach reports a new STANDUP for yesterday (2023-05-12):

Progress: - TypeNameTest and RuntimeErrorsTest: f2df70c

• review of Dmitry's alternative implementation: SuggestionBuilder needs to send ascribedType of constructor parameters #6655
• if_then_else evaluated: Broken redefinition of variable inside if-then expression #6211
• review of Meta improvements: Improving widgets for take/drop #6641 (review)
• bookclub: trying to login, find projects, etc: https://discord.com/channels/401396655599124480/1106533940086448258/1106553375136436285
• cannot ./run gui watch with local project-manager
• Initializiting project... Display Initializing project... message when initializing project #6661
• standup & discussions & spreading GraalVM news It should be finished by 2023-05-12.

Next Day: Finding a way to join Dmitry's #6655 and my #6584

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