symbolizeExpr crashes due to error when Unknown language

[didrikmunther]

Example:

Unknown language in symbolize: MLangAst

Caused by (stdlib/mexpr/symbolize.mc):

sem symLookupError lkup =| ident ->
    errorSingle lkup.info
      (join ["Unknown ", lkup.kind, " in symbolize: ", nameGetStr ident])

This makes it impossible to provide language support, since the compiler crashes when typing the language fragment. A better solution would be to change the symbolizeExpr signature to something like this:

syn SymbolizeError =
  | UnknownLanguageError String
  | OtherError String
  
  sem symbolizeExpr : SymEnv -> Expr -> Result [(Info, SymbolizeWarning)] [(Info, SymbolizeError)] Expr

And then changing all symbolizeExpr to use results, e.g.:

lang LetSym = Sym + LetAst + AllTypeAst
  sem symbolizeExpr (env : SymEnv) =
  | TmLet t ->
    match symbolizeTyAnnot env t.tyAnnot with (tyVarEnv, tyAnnot) in
    match setSymbol env.currentEnv.varEnv t.ident with (varEnv, ident) in
    let body = symbolizeExpr (symbolizeUpdateTyVarEnv env tyVarEnv) t.body in
    let inexpr = symbolizeExpr (symbolizeUpdateVarEnv env varEnv) t.inexpr in
    result.map2 (
      lam body. lam inexpr.
        TmLet {t with ident = ident,
                  tyAnnot = tyAnnot,
                  body = body,
                  inexpr = inexpr}
    ) body inexpr