Expr

type Expr =
  | { readonly op: 'lit'; readonly type: ShaderType; readonly value: number | boolean }
  | { readonly op: 'constref'; readonly type: ShaderType; readonly name: string }
  | { readonly op: 'overrideref'; readonly type: ShaderType; readonly name: string }
  | { readonly op: 'externref'; readonly type: ShaderType; readonly name: string }
  | {
      readonly op: 'param';
      readonly type: ShaderType;
      readonly name: string;
      /** Where this was written, when it is the TARGET of an assignment — the lvalue a
       *  statement is about to write, so a debugger can highlight what changes rather than the
       *  whole line. A read of the same name carries none in this increment. Read it with
       *  {@link sourceSpanOf}. */
      readonly span?: SourceSpan;
    }
  | {
      readonly op: 'varref';
      readonly type: ShaderType;
      readonly name: string;
      /** Where this was written, when it is the TARGET of an assignment — the lvalue a
       *  statement is about to write, so a debugger can highlight what changes rather than the
       *  whole line. A read of the same name carries none in this increment. Read it with
       *  {@link sourceSpanOf}. */
      readonly span?: SourceSpan;
    }
  | {
      readonly op: 'binop';
      readonly type: ShaderType;
      readonly bop: BinOp;
      readonly a: Expr;
      readonly b: Expr;
    }
  | { readonly op: 'unop'; readonly type: ShaderType; readonly a: Expr }
  | {
      readonly op: 'compare';
      readonly type: ShaderType;
      readonly cop: CmpOp;
      readonly a: Expr;
      readonly b: Expr;
    }
  | {
      readonly op: 'logical';
      readonly type: ShaderType;
      readonly lop: LogOp;
      readonly a: Expr;
      readonly b: Expr;
    }
  | {
      readonly op: 'call';
      readonly type: ShaderType;
      readonly fn: string;
      readonly args: readonly Expr[];
      readonly declRef?: FuncDecl;
      /** Where this call was written, when it is the outermost call node lowered from a
       *  `ts.CallExpression`. Stepping into a helper has to tell two calls in one statement
       *  apart, which is why this expression kind carries a span at all. A call the front end
       *  synthesises while expanding one (the `random` hash, an array higher-order function,
       *  a `Math.*` expansion, a numeric cast) carries none, because it was written nowhere.
       *  Read it with {@link sourceSpanOf}. */
      readonly span?: SourceSpan;
      /** On a `console.*` call whose author wrote a string literal among the arguments: every
       *  argument in the order written, a string for a label and a number for the index of a
       *  value in `args`. A label has no GPU representation and is never an `Expr`; it lives
       *  here, on the host's side of the call (Rule 7.8, surface §66). Absent when every
       *  argument is a value, which is then `args` in order. */
      readonly labels?: readonly (string | number)[];
    }
  | {
      readonly op: 'member';
      readonly type: ShaderType;
      readonly base: Expr;
      readonly field: string;
    }
  | { readonly op: 'construct'; readonly type: ShaderType; readonly args: readonly Expr[] }
  | {
      readonly op: 'select';
      readonly type: ShaderType;
      readonly cond: Expr;
      readonly ifTrue: Expr;
      readonly ifFalse: Expr;
    }
  | {
      readonly op: 'index';
      readonly type: ShaderType;
      readonly base: Expr;
      readonly idx: Expr;
      /** Where this was written, when it is the TARGET of an assignment — the lvalue a
       *  statement is about to write, so a debugger can highlight what changes rather than the
       *  whole line. A read of the same name carries none in this increment. Read it with
       *  {@link sourceSpanOf}. */
      readonly span?: SourceSpan;
    }
  | {
      readonly op: 'matchExpr';
      readonly type: ShaderType;
      readonly scrutinee: Expr;
      readonly cases: ReadonlyArray<readonly [number, Expr]>;
      readonly default: Expr;
    };