{"id":24794,"date":"2022-12-05T19:46:22","date_gmt":"2022-12-05T14:16:22","guid":{"rendered":"https:\/\/jassweb.com\/solved\/solved-couldnt-match-type-synonym-with-either\/"},"modified":"2022-12-05T19:46:22","modified_gmt":"2022-12-05T14:16:22","slug":"solved-couldnt-match-type-synonym-with-either","status":"publish","type":"post","link":"https:\/\/jassweb.com\/solved\/solved-couldnt-match-type-synonym-with-either\/","title":{"rendered":"[Solved] Couldn’t match type Synonym with Either"},"content":{"rendered":"

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Either<\/code> isn’t simply a union of types; it’s a tagged<\/em> union, which means every value has to explicitly specify which “side” of the type the wrapped value occurs on. Here’s an example, (with a Show<\/code> instance derived for your Card<\/code> type):<\/p>\n

*Main> Card Hearts Jack\n\n<interactive>:3:13: error:\n    \u2022 Couldn't match type \u2018Court\u2019 with \u2018Either Pip Court\u2019\n      Expected type: Rank\n        Actual type: Court\n    \u2022 In the second argument of \u2018Card\u2019, namely \u2018Jack\u2019\n      In the expression: Card Hearts Jack\n      In an equation for \u2018it\u2019: it = Card Hearts Jack\n<\/code><\/pre>\n
Card<\/code> expects an argument of type Either Pip Court<\/code>, but you gave it a plain Court<\/code> value. With an explicit wrap:<\/p>\n
*Main> Card Hearts (Right Jack)\nCard Hearts (Right Jack)\n<\/code><\/pre>\n
The same applies to creating a numbered card:<\/p>\n
*Main> Card Hearts 3\n\n<interactive>:5:13: error:\n    \u2022 No instance for (Num Rank) arising from the literal \u20183\u2019\n    \u2022 In the second argument of \u2018Card\u2019, namely \u20183\u2019\n      In the expression: Card Hearts 3\n      In an equation for \u2018it\u2019: it = Card Hearts 3\n*Main> Card Hearts (Left 3)\nCard Hearts (Left 3)\n<\/code><\/pre>\n
You don’t need to change the definition of snap<\/code>, because there is already an Eq<\/code> instance for Either<\/code> as long as both wrapped types have Eq<\/code> instances themselves; it considers any Right<\/code> value unequal to any Left<\/code> value, and matching Right<\/code> or Left<\/code> values are equal if the wrapped values are equal.<\/p>\n
*Main> snap (Card Hearts (Right Jack)) (Card Hearts (Left 3))\n\"...\"\n*Main> snap (Card Hearts (Right Jack)) (Card Spades (Right Jack))\n\"SNAP\"\n<\/code><\/pre>\n<\/p><\/div>\n
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solved Couldn’t match type Synonym with Either <\/p>\n","protected":false},"excerpt":{"rendered":"
[ad_1] Either isn’t simply a union of types; it’s a tagged union, which means every value has to explicitly specify which “side” of the type the wrapped value occurs on. Here’s an example, (with a Show instance derived for your Card type): *Main> Card Hearts Jack <interactive>:3:13: error: \u2022 Couldn’t match type \u2018Court\u2019 with \u2018Either … Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[320],"tags":[5228,1150,2420,438],"class_list":["post-24794","post","type-post","status-publish","format-standard","hentry","category-solved","tag-either","tag-haskell","tag-pattern-matching","tag-types"],"yoast_head":"\n[Solved] Couldn't match type Synonym with Either - JassWeb<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/jassweb.com\/solved\/solved-couldnt-match-type-synonym-with-either\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"[Solved] Couldn't match type Synonym with Either - JassWeb\" \/>\n<meta property=\"og:description\" content=\"[ad_1] Either isn’t simply a union of types; it’s a tagged union, which means every value has to explicitly specify which “side” of the type the wrapped value occurs on. 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