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`RoleMismatch` Error

errors/RoleMismatch.md

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Original Source

RoleMismatch Error

Example

purescript
module ShortFailingExample where

data D a = D a
type role D phantom
purescript
module ShortFailingExample where

data D f a = D (f a)
type role D representational representational

Cause

A parameter was assigned a role more permissive than required.

Fix

  • Remove or strengthen the role declaration:
diff
 data D a = D a
-type role D phantom
+type role D representational
diff
 data D f a = D (f a)
-type role D representational representational
+type role D representational nominal