{"id":940,"date":"2022-08-30T15:09:43","date_gmt":"2022-08-30T15:09:43","guid":{"rendered":"https:\/\/unknownerror.org\/index.php\/2013\/11\/09\/f-can-someone-explain-my-compiler-error-collection-of-common-programming-errors\/"},"modified":"2022-08-30T15:09:43","modified_gmt":"2022-08-30T15:09:43","slug":"f-can-someone-explain-my-compiler-error-collection-of-common-programming-errors","status":"publish","type":"post","link":"https:\/\/unknownerror.org\/index.php\/2022\/08\/30\/f-can-someone-explain-my-compiler-error-collection-of-common-programming-errors\/","title":{"rendered":"F#: Can someone explain my compiler error?-Collection of common programming errors"},"content":{"rendered":"

Let’s try to reproduce how the compiler tries to infer types here.<\/p>\n

let rec Foo(a,b) =\n    match a () with\n    | None -> Some(b)\n    | Some(c) -> Some(Foo(c,b))\n<\/code><\/pre>\n
“Ok, so I see a ()<\/code>. a<\/code> must be a function from unit<\/code> to some type, I don’t know which one yet. I’ll call it 'a<\/code>.”<\/p>\n
a : unit -> 'a\n<\/code><\/pre>\n
“The result of a ()<\/code> is matched with None<\/code>\/Some<\/code> patterns. So 'a<\/code> must be a 'b option<\/code> and c<\/code> has the type 'b<\/code>.” (Again, 'b<\/code> stands for an unknown, as of yet, type).<\/p>\n
a : unit -> 'b option\n\u0441 : 'b\n<\/code><\/pre>\n
“No functions or methods are called on b<\/code> (except Some<\/code>, which doesn’t narrow the type down, and Foo<\/code>, the type of which we don’t know so far). I’ll denote its type by 'c<\/code>.”<\/p>\n
a : unit -> 'b option\nb : 'c\nc : 'b\n<\/code><\/pre>\n
“Foo<\/code> returns Some(b)<\/code> in one of the branches, so the return type must be 'c option<\/code>.”<\/p>\n
Foo : (unit -> 'b option) * 'c -> 'c option\n<\/code><\/pre>\n
“Am I done yet? No, I need to check that all types in the expression make sense. Let’s see, in the Some(c)<\/code> case, Some(Foo(c,b))<\/code> is returned. So Foo(c,b) : 'c<\/code>. Since Foo<\/code> returns an option<\/code>, I know 'c<\/code> must be 'd option<\/code> for some 'd<\/code>, and b : 'd<\/code>. Wait, I already have b : 'c<\/code>, that is, b : 'd option<\/code>. 'd<\/code> and 'd option<\/code> have to be the same type, but this is impossible! There must be an error in the definition. I need to report it.” So it does.<\/p>\n
Originally posted 2013-11-09 23:04:41. <\/small><\/p>","protected":false},"excerpt":{"rendered":"
Let’s try to reproduce how the compiler tries to infer types here. let rec Foo(a,b) = match a () with | None -> Some(b) | Some(c) -> Some(Foo(c,b)) “Ok, so I see a (). a must be a function from unit to some type, I don’t know which one yet. I’ll call it ‘a.” a […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-940","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/posts\/940","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/comments?post=940"}],"version-history":[{"count":0,"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/posts\/940\/revisions"}],"wp:attachment":[{"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/media?parent=940"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/categories?post=940"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/unknownerror.org\/index.php\/wp-json\/wp\/v2\/tags?post=940"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}