- [x] downgrade stage
- [ ] resolve stage
  - [ ] dynamic attr resolution
  - [ ] import resolution
    - [ ] static path resolution
- [ ] eval stage
  - [ ] dynamic path resolution
  - [ ] graph update
  - [ ] stack storage

阻拦变量解析的是 `with import` 而不是一般的 `with`
解析阶段可以解决除涉及非字符串常量拼接路径导入之外的问题

静态分析图与求值期图分离，动态生成求值期图

依赖类型：
1. 强依赖 (x!)
  - builtins.toString x => x!
  - x + y => x! y!
  - { x = 1; ${sym} = 2; } => sym!
2. 弱依赖 a.k.a. thunk (x?)
  - builtins.seq x y => x! y?
3. 递归依赖 (x!!)
  - builtins.deepSeq x y => x!! y?

e.g.
- let a? = { inherit a?; }; in a! => a!
  - a! => { a = <THUNK> }
- f: let x? = f! x?; in x! => f! x!
- let ret? = (self: { x? = 1; y? = self.x! + 1; }) ret?; in ret.y! => ret! x! y!

工作流程：
1. string -> AST
2. AST -> HIR (alloc ExprId)
3. HIR -> LIR (resolve var, build graph)
4. LIR -> Value

```mermaid
flowchart TD
  Start([Context::eval]) --> Eval[EvalContext::eval]
  Eval --> AddNode[add node to graph]
  AddNode --> CheckType{check expression type}

  CheckType -->|AttrSet| A_EvalKeys[eval keys]
  A_EvalKeys --> A_Construct[construct attrset]
  A_Construct --> A_Return[return attrset]
  A_Return --> End
  A_EvalKeys -.->|eval| Eval

  CheckType -->|List| L_Construct[construct list]
  L_Construct --> L_Return[return list]
  L_Return --> End

  CheckType -->|HasAttr| HA_ForceAttrSet[force attrset]
  HA_ForceAttrSet --> HA_ForceAttrPath[force attrpath]
  HA_ForceAttrPath --> HA_Return[return bool]
  HA_Return --> End
  HA_ForceAttrSet -.->|eval| Eval
  HA_ForceAttrPath -.->|eval| Eval

  CheckType -->|BinOp| B_ForceLeft[force left operand]
  B_ForceLeft --> B_ForceRight[force right operand]
  B_ForceRight --> B_Apply[apply operator]
  B_Apply --> B_Return[return value]
  B_Return --> End
  B_ForceLeft -.->|eval| Eval
  B_ForceRight -.->|eval| Eval

  CheckType -->|UnOp| U_ForceOperand[force operand]
  U_ForceOperand --> U_Apply[apply operator]
  U_Apply --> U_Return[return value]
  U_Return --> End
  U_ForceOperand -.->|eval| Eval

  CheckType -->|Select| S_ForceAttrSet[force attrset]
  S_ForceAttrSet --> S_ForceAttrPath[force attrpath]
  S_ForceAttrPath --> S_CallEval[call eval on value]
  S_CallEval --> S_Return[return value]
  S_Return --> End
  S_ForceAttrSet -.->|eval| Eval
  S_ForceAttrPath -.->|eval| Eval
  S_CallEval -.-> Eval

  CheckType -->|If| I_ForceCond[force condition]
  I_ForceCond --> I_Cond{condition}
  I_Cond -->|true| I_ForceConsq[force consequence]
  I_Cond -->|false| I_ForceAlter[force alternative]
  I_ForceConsq --> I_CallEvalConsq[call eval on consequence]
  I_ForceAlter --> I_CallEvalAlter[call eval on alternative]
  I_CallEvalConsq --> I_ReturnConsq[return value]
  I_CallEvalAlter --> I_ReturnAlter[return value]
  I_ReturnConsq --> End
  I_ReturnAlter --> End
  I_ForceCond -.->|eval| Eval
  I_ForceConsq -.->|eval| Eval
  I_ForceAlter -.->|eval| Eval
  I_CallEvalConsq -.-> Eval
  I_CallEvalAlter -.-> Eval

  CheckType -->|Call| C_RegArg[register argument]
  C_RegArg --> C_ForceBody[force body]
  C_ForceBody --> C_CallEval[call eval on body]
  C_CallEval --> C_Return[return value]
  C_Return --> End
  C_ForceBody -.->|eval| Eval
  C_CallEval -.-> Eval

  CheckType -->|With| W_EnterWith[enter with]
  W_EnterWith --> W_ForceBody[force body]
  W_ForceBody --> W_CallEval[call eval on body]
  W_CallEval --> W_Return[return value]
  W_Return --> End
  W_ForceBody -.->|eval| Eval
  W_CallEval -.-> Eval

  CheckType -->|Assert| As_ForceCond[force condition]
  As_ForceCond --> As_Cond{condition}
  As_Cond -->|true| As_ForceBody[force body]
  As_Cond -->|false| As_Throw[throw Catchable]
  As_ForceBody --> As_CallEval[call eval on body]
  As_CallEval --> End
  As_ForceCond -.->|eval| Eval
  As_ForceBody -.->|eval| Eval
  As_CallEval -.-> Eval

  CheckType -->|ConcatStrings| CS_ForceParts[force string parts]
  CS_ForceParts --> CS_Construct[construct string]
  CS_Construct --> End
  CS_ForceParts -.->|eval| Eval

  CheckType -->|Const| Co_Return[return constant]
  Co_Return --> End

  CheckType -->|Str| St_Return[return string]
  St_Return --> End

  CheckType -->|Var| V_Lookup[lookup var]
  V_Lookup --> V_Return[return value]
  V_Return --> End

  CheckType -->|Arg| Ar_Lookup[lookup arg]
  Ar_Lookup --> End

  CheckType -->|Func| F_Construct[construct function]
  F_Construct --> End

  CheckType -->|StrictRef| SR_Eval[eval referenced expr]
  SR_Eval --> Eval

  CheckType -->|LazyRef| LR_Resolve[resolve dynamic variable lookups of the referenced expr]
  LR_Resolve --> LR_Contruct[construct lazy reference]
  LR_Contruct --> End

  End([return result])
```