Generating fresh names in a source plugin

[Kai Prott]

Hi,

what I have used In the past (and that I am fairly confident works) is exactly what you say: use `uniqFromTag` or `mkSplitUniqSupply`.

According to the documentation, "the payload part of the Uniques allocated from this UniqSupply are guaranteed distinct wrt all other supplies, regardless of their 'tag'."
(Assuming the unique does not overflow it's number of bits)

As far as I know, end users should not get to see a unique and it should not influence compilation, so the Tag is only relevant for plugin/GHC devs for debugging. 

Also just curious: why do you need to construct a unique? Before the GHC renamer a plugin can get away with generating essentially strings that are "unique" and use them in `mkUnqual` or similar. 
Most of the time, I just generate a `RdrName` with a prefix that would not be allowed in source code with an incrementing suffix that I manage myself. 
I have not checked if a plugin-generated name has to be a valid Haskell name nowadays. However, it worked in the past.

And just for completeness if someone else stumbles across this: After the GHC renamer in `CoreM`, `TcM` (and others) one can just use `getUniqueM` or `getUniqueSupplyM`.

Best wishes,
Kai Prott 

On August 2, 2025 7:08:10 PM GMT+02:00, Wolfgang Jeltsch <wolfgang at well-typed.com> wrote:
>Hello, again!
>
>After some more digging, I came to the conclusion that `uniqFromTag` is
>likely what I should use. However, what exactly is the tag for?
>
>The note „Uniques and tags“ only says the following:
>
>> The tag […] is typically used to make it easier to distinguish uniques
>> constructed by different parts of the compiler.
>
>Who distinguishes uniques using tags: the compiler, compiler developers
>when reading debugging output, or end users? Does the choice of tag
>influence the behavior of the compiler other than with respect to
>messages?
>
>The documentation of `mkSplitUniqSupply` says that the tag “is mostly
>cosmetic”. If it is only *mostly* cosmetic, what aspects of it are not
>cosmetic?
>
>In the end, how do I choose a tag for uniques generated by a plugin? Is
>the choice essentially irrelevant? Do I have to prevent clashes with
>tags used by GHC or other plugins?
>
>Looking forward to insights. 🙂
>
>All the best,
>Wolfgang
>
>Am Samstag, dem 02.08.2025 um 16:16 +0300 schrieb Wolfgang Jeltsch:
>> Hello!
>> 
>> I want to generate AST fragments in a source plugin, and for this I
>> need to generate fresh (local) names. My current approach is to use
>> `mkSystemName`, but it, like many of the other name-generating
>> operations, needs a value of type `Unique`. How can I generate such
>> values? It seems that the `Hsc` monad doesn’t carry a value of type
>> `UniqueSupply` with it.
>> 
>> All the best,
>> Wolfgang
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