See @gibiansky/IHaskell#367

show-instance-bug-development.ipynb

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    ":set -XTypeFamilies\n",
    ":set -XDataKinds"
   ]
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   "cell_type": "code",
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   "source": [
    "import Data.Metrology\n",
    "import Data.Metrology.SI\n",
    "import Data.Metrology.Show"
   ]
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    "data AngleDim = AngleDim\n",
    "data Radian = Radian\n",
    "\n",
    "instance Dimension AngleDim where\n",
    "  type DimFactorsOf AngleDim = '[]\n",
    "\n",
    "type instance DefaultUnitOfDim AngleDim = Radian\n",
    "\n",
    "instance Unit Radian where\n",
    "  type BaseUnit Radian = Canonical\n",
    "  type DimOfUnit Radian = AngleDim\n",
    "  type UnitFactorsOf Radian = '[]"
   ]
  },
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      "text/plain": [
       "0.3"
      ]
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   "source": [
    "0.3 % Radian"
   ]
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  {
   "cell_type": "code",
   "execution_count": 5,
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   "source": [
    "instance Show Radian where\n",
    "  show _ = \"rad\""
   ]
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       "0.3"
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   "source": [
    "0.3 % Radian"
   ]
  },
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   "execution_count": null,
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