Haskell,语法错误

Question

temperatura :: Float->Float
temperatura qCalor
    | qCalor == 0 = 10
    | 0 < qCalor < 3 = 30--fTem1
    | 3 <= qCalor <= 9 = 50
    | qCalor > 9 = 60--fTemp2
    | 15 <= qCalor <= 24 = 150
    | 24 < qCalor <= 27 = 170--fTemp3
    | otherwise = "Nao existe temperatura correspondente a esse calor no grafico!"

优先级解析错误为什么会这样？

Answer 1

首先,在提出问题时,发布实际错误是有帮助的.这是我得到的:

test.hs:3:7:
    Precedence parsing error
        cannot mix `<' [infix 4] and `<' [infix 4] in the same infix expression

test.hs:4:7:
    Precedence parsing error
        cannot mix `<=' [infix 4] and `<=' [infix 4] in the same infix expression

test.hs:6:7:
    Precedence parsing error
        cannot mix `<=' [infix 4] and `<=' [infix 4] in the same infix expression


test.hs:7:7:
    Precedence parsing error
        cannot mix `<' [infix 4] and `<=' [infix 4] in the same infix expression

现在,问题基本上是<(和其他比较运算符)实际上只是二进制函数 - 带有两个参数的函数.编译器告诉您它无法知道如何在表达式中放置括号,因为这些函数具有相同的优先级.举个例子,这个:

| 0 < qCalor < 3 = 30

编译器不知道是否就意味着(0 < qCalor) < 3或0 < (qCalor < 3).在任何情况下,该行都没有合理的输入.

我建议使用像(0 < qCalor) && (qCalor < 3)(或者更好)这样的函数(可能有一个内置的函数):

betweenNums a b c = (a < b) && (b < c)