# Can \`where\` constrain type parameters that are instances, like \`where {T isa Symbol}\`

**URL:** <https://discourse.julialang.org/t/can-where-constrain-type-parameters-that-are-instances-like-where-t-isa-symbol/81443>\
**Category:** General Usage\
**Tags:** parametric-types, parametric-methods\
**Created:** [May 22, 2022, 12:31am UTC](https://discourse.julialang.org/t/can-where-constrain-type-parameters-that-are-instances-like-where-t-isa-symbol/81443 "2022-05-22T00:31:22Z")\
**Posts on this page:** 1\
**Showing post:** 7

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**Author:** ![Benny](https://avatars.discourse-cdn.com/v4/letter/b/49beb7/32.png) [@Benny](https://discourse.julialang.org/u/Benny)\
**Post date:** [May 22, 2022, 6:56pm UTC](https://discourse.julialang.org/t/can-where-constrain-type-parameters-that-are-instances-like-where-t-isa-symbol/81443/7 "2022-05-22T18:56:43Z")

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I don’t understand the type-speak stuff, but it does make sense that `where` isn’t working on types and values directly, so something like an `isa` isn’t possible. This question popped into my head because of the last two points in [this comment](https://discourse.julialang.org/t/understand-why-type-stability-depends-on-number-of-methods/81434/3); it was possible to jerry-rig a type-stability optimization by annotating a parametric method’s return type, but it seemed more natural to attempt to constrain the parameter based on the methods the parametric method would replace.

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