enforce that we don't add vars twice to env Gamma

[womeier]

Currently, we don't check if a var is already in Gamma before adding it. We could check and throw an error if it is already present.

[mkarup]

Isn't it okay for the same variable name to be in there more than once, since we pick the first one we encounter, if it's there? Is it in correlation with the constructors, where you can "overwrite" the definition of one of the constructors?

[womeier]

this was in reaction to

Since constructors and data types are part of the "global environment", I "inject" the constructors into the environment as a preprocessing step. I did this to avoid changing typechecking too much, e.g. adding an extra environment for data types/ constructors and passing that around. Do you think this preprocessing should be more incorporated into the typechecking, e.g. so checking that the definition of the constructors/ data types is reasonable, and that there are no name clashes, is part of typechecking?

Thought, if we just raised an exception for duplicate entries, this would be a simple solution for name clashes.
In case we decide, we want to prevent that, just opened it since you brought it up there.

[mkarup]

this was in reaction to

Since constructors and data types are part of the "global environment", I "inject" the constructors into the environment as a preprocessing step. I did this to avoid changing typechecking too much, e.g. adding an extra environment for data types/ constructors and passing that around. Do you think this preprocessing should be more incorporated into the typechecking, e.g. so checking that the definition of the constructors/ data types is reasonable, and that there are no name clashes, is part of typechecking?

Thought, if we just raised an exception for duplicate entries, this would be a simple solution for name clashes. In case we decide, we want to prevent that, just opened it since you brought it up there.

I think it's fine for regular variables that can be let bound. I've updated the data type PR to try to account for the issue. This should prevent 'overriding' constructor names