[英]How does one avoid “Value restriction” errors with F#'s Seq.cast?
I see that Seq has a cast function from IEnumerable to Seq, but how do I get it to work? 我看到Seq有一个从IEnumerable到Seq的强制转换功能,但是如何让它工作呢?
open System.Text.RegularExpressions;;
let v = Regex.Match("abcd", "(ab)");;
Seq.cast (v.Captures);;
This produces, 这会产生,
error FS0030: Value restriction. 错误FS0030:值限制。 The value 'it' has been inferred to have generic type val it : seq<'_a>
值'it'被推断为具有泛型类型val:seq <'_ a>
Either define 'it' as a simple data term, make it a function with explicit arguments or, if you do not intend for it to be generic, add a type annotation. 将'it'定义为一个简单的数据项,使其成为具有显式参数的函数,或者,如果您不打算将其作为泛型,则添加类型注释。
Be explicit about the type: 明确类型:
Seq.cast<Match> (v.Captures)
Otherwise cast
needs context to infer the right return type, and on a line by itself like there, there is no such context for type inference to use. 否则
cast
需要上下文来推断正确的返回类型,像有一个行本身没有这样的上下文类型推断使用。
(This line converts the non-generic IEnumerable
into a generic IEnumerable<Match>
aka seq<Match>
.) (此行将非通用
IEnumerable
转换为通用IEnumerable<Match>
aka seq<Match>
。)
There are actually two ways to specify the type you want to get. 实际上有两种方法可以指定您想要获得的类型。 Brian posted how to do this by explicitly specifying the type parameter to a function:
Brian通过显式指定函数的type参数发布了如何执行此操作:
let res = Seq.cast<Match> v.Captures
The other option is to use type annotations which can be placed around any F# expression and specify the type of the expression - this way you can hint the compiler type inferrence (by saying that some expression has a particular type). 另一个选项是使用类型注释 ,它可以放在任何F#表达式周围并指定表达式的类型 - 这样就可以提示编译器类型推理(通过说某些表达式具有特定类型)。 If you provide the information in some clever way, the compiler will be able to figure out what the type parameter to
Seq.cast
should be. 如果以一种聪明的方式提供信息,编译器将能够确定
Seq.cast
的类型参数应该是什么。 A few examples: 几个例子:
// By specifying type of the value
let (res:seq<Match>) = Seq.cast v.Captures
// By specifying return type of a function
let getCaptures () : seq<Match> =
// ...
Seq.cast v.Captures
// By specifying element type when iterating over the result
for (m:Match) in Seq.cast v.Captures do
// ...
From all of the options, I think that the Brians (explicit) and my second (return type of a function) are those that are the most idiomatic, however, you can choose any option you find the most readable. 从所有选项中,我认为Brians(显式)和我的第二个(函数的返回类型)是那些最惯用的选项,但是,您可以选择任何您认为最具可读性的选项。
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