Warn型クラス
https://github.com/purescript/documentation/blob/master/guides/Custom-Type-Errors.md
型シグネチャ部分でwarning message変えられるのおもろい
https://liamgoodacre.github.io/purescript/warnings/2017/01/17/purescript-warn-type-class.html
https://pursuit.purescript.org/builtins/docs/Prim.TypeError
https://github.com/purescript/documentation/blob/master/guides/Custom-Type-Errors.md
関連
Haskellのカスタム型エラー