将元组列表打印到标准输出"表"中

Question

我有一个元组列表[("hi", 1), ("yo", 2)];，我想把这个列表'转换'成一个表，使用sml中的标准输出print s;成表格式。

例如，上面的元组列表将产生如下输出:

------------
| "hi" | 1 |
------------
etc...
------------

我有这个代码，但是它给了我一个错误:

fun outputTable [] = print "Nothing! n"
  | outputTable ((hd, n)::tl) =
      print "-----------"
      print "| "^hd^" | "^n^" |"
      print "-----------"
      outputTable tl;

错误如下:

stdIn:15.2-17.21 Error: operator is not a function [tycon mismatch]  
operator: unit
  in expression:
    (print "-----------") print
stdIn:15.2-17.21 Error: operator is not a function [tycon mismatch]
operator: string
  in expression:
    " |" outputTable
stdIn:13.5-17.21 Error: right-hand-side of clause doesn't agree with function result type [tycon mismatch]
expression: string
result type:  unit
in declaration:
    outputTable =
      (fn nil =print "Nothing!"
        | :: (<pat>,<pat>) =<exp^ <exp^ <exp<exp>)

Answer 1

要在SML中使用多个表达式，必须将多行括在括号中，如下所示:

(expr 1; expr2; ...; exprn);

因此，为了修正我的错误，我只是对我的打印行使用了这个形式:

fun outputTable [] = []
  | outputTable ((hd, n)::tl) =
      (print "------------------n";
      print ("| " ^ hd ^ " | " ^ Int.toString(n) ^ " |n");
      print "-------------------n";
      outputTable tl);