OCaml核心操作出现未绑定值错误的原因咨询

编写OCaml代码时遇到异常错误：核心语言元素raise、=、+均被提示为「未绑定值」，相关代码如下：

原第一段代码 #

type id = string

let rec lookup (id: id) (storage: (id * 'a) list): 'a =
  match storage with
  | [] -> raise Not_found
  | (id', element)::storage' -
    if id = id'
    then element
    else lookup id storage'

原第二段代码 #

let rec interpretExpression
  (expression: expression)
  (valueStorage: value_storage)
  (functionStorage: function_storage)
: int =
  match expression with
  | Id id -> lookup id valueStorage
  | Int value -> value
  | Add (expression1, expression2) -
    let value1 = interpretExpression expression1 valueStorage functionStorage
    and value2 = interpretExpression expression2 valueStorage functionStorage
    in value1 + value2
  | FuncApp id -
    let expression' = lookup id functionStorage
    in interpretExpression expression' valueStorage functionStorage

问题根源 #

代码存在模式匹配箭头书写不完整的语法错误：

• 第一段中| (id', element)::storage' -的箭头仅写了-，正确应为->；
• 第二段中| Add (expression1, expression2) -、| FuncApp id -同样缺失了箭头的>部分。

OCaml要求模式匹配分支必须用->分隔模式与执行代码，箭头书写不完整会导致编译器无法正确解析语法结构，进而将后续代码中的核心语法元素误判为未绑定标识符。

修正后的代码 #

第一段修正版

type id = string

let rec lookup (id: id) (storage: (id * 'a) list): 'a =
  match storage with
  | [] -> raise Not_found
  | (id', element)::storage' ->
    if id = id'
    then element
    else lookup id storage'

第二段修正版

let rec interpretExpression
  (expression: expression)
  (valueStorage: value_storage)
  (functionStorage: function_storage)
: int =
  match expression with
  | Id id -> lookup id valueStorage
  | Int value -> value
  | Add (expression1, expression2) ->
    let value1 = interpretExpression expression1 valueStorage functionStorage
    and value2 = interpretExpression expression2 valueStorage functionStorage
    in value1 + value2
  | FuncApp id ->
    let expression' = lookup id functionStorage
    in interpretExpression expression' valueStorage functionStorage

修正箭头后，编译器可正常解析语法，核心元素的未绑定值错误会自动消失。

内容的提问来源于stack exchange，提问作者Little Helper