ocaml类型不匹配单元与列表

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为此签名

val chooser: string list * string list -> string list

和此实现

let rec chooser (inputList, trueList) = match inputList with [] -> [] | iH::iT -> if (List.hd trueList)="True" then iH::(chooser iT List.tl trueList)

我收到以下错误:

错误:此变量表达式应具有类型单位 构造函数::不属于单位

Error: This variant expression is expected to have type unit The constructor :: does not belong to unit

我做错了什么?

推荐答案

不包含else的if ... then的结果必须为unit，因为该值将为()(类型为)表达式为假时.

The result of if ... then with no else has to be unit, because the value will be () (the value of type unit) when the expression is false.

换句话说，您需要为if使用else部分来获取所需的类型.比较为假时，该值应该是什么?

In other words, you need an else part for your if to get the type you want. What should the value be when the comparison is false?