为什么会出现这些错误，我该如何解决? [英] Why am I getting these errors, and how can I solve them?

问题描述

我正在用Standard ML编写解释器，但是该函数的语法有问题，我无法弄清楚出了什么问题.

I'm writing an interpreter in Standard ML but I'm having trouble with my the syntax in this function, and I cannot figure out what's wrong.

以下是相关代码:

| eval (rho, SetExp (name, value)) =
    (case rhoContains rho name of
        true    => rhoSet rho name value (rho, value)
    |   false   => globalSet (name, value))


fun rhoSet [] key value = [(key, value)]
  | rhoSet ((elt as (k, v)) :: tail) key value =
        if key = k then (key, value) :: tail else elt :: rhoSet tail key value


fun rhoContains rho name =
    case rhoGet rho name of SOME _ => true | NONE => false

这是SetExp的来源:

This is where SetExp comes from:

datatype expression =
    SetExp of (string * expression)

运行此命令会给我列出很多错误，但是我认为这是相关的部分.第62行是eval中以true开头的行:

Running this gives me a long list of errors but I think this is the relevant section. Line 62 is the line that starts with true in eval:

eval.sml:62: error: Type error in function application.
  Function: rhoSet rho name value : (string * expression) list
  Argument: (rho, value) : (string * expression) list * expression
  Reason: Value being applied does not have a function type

推荐答案

您要将太多参数传递给rhoSet-删除尾随对.

You're passing too many arguments to rhoSet - remove the trailing pair.

| eval (rho, SetExp (name, value)) =
    (case rhoContains rho name of
        true    => rhoSet rho name value
    |   false   => globalSet (name, value))

您还可以通过条件使此内容更具可读性:

You can also make this more readable with a conditional:

| eval (rho, SetExp (name, value)) =
    if rhoContains rho name
    then rhoSet rho name value
    else globalSet (name, value)

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