如何编写SML函数统计布尔列表中true的出现次数？

Hey there! I see where the issue is with your function—let's break it down and fix it.

The Problem in Your Code #

Your current code uses foldl correctly in terms of the overall approach, but the anonymous function you passed to foldl is missing a critical part: the else branch of your if expression.

In SML, every if expression must have both a then and else branch because it's an expression that needs to return a value of a consistent type. Right now, when x is false, your function doesn't specify what to return, which causes a type mismatch error (and prevents the code from compiling properly).

Your original code:

fun truecount(lst) = foldl (fn (x,y) => if x=true then y+1) 0 lst;

The Fixed Version #

We just need to add an else clause that returns the current count y when x is false. Also, since x is already a boolean value, we can simplify x=true to just x for cleaner code:

fun truecount(lst) = foldl (fn (x,y) => if x then y + 1 else y) 0 lst;

How This Works #

• foldl iterates over the list from left to right, using 0 as the initial count.
• For each element x in the list:
  • If x is true, we increment the accumulated count y by 1.
  • If x is false, we keep the accumulated count y the same.
• At the end of the iteration, foldl returns the total count of true values.

Alternative Recursive Implementation #

If you're curious, here's a straightforward recursive version of the same function, which might help clarify the logic further:

fun truecount [] = 0
  | truecount (true::rest) = 1 + truecount rest
  | truecount (false::rest) = truecount rest;

This recursive approach pattern-matches on the list:

• Empty list returns 0 (base case).
• If the head is true, add 1 and recurse on the rest of the list.
• If the head is false, just recurse on the rest of the list without changing the count.

内容的提问来源于stack exchange，提问作者B. Williams