haskell - 拥抱不要让脚本文件使用关键字“让”

Question

我将此文件作为模块加载到 Hugs，但收到此错误：

错误文件:.\Hugs.hs:38 - 输入中的语法错误（意外的关键字“let”）

data MetricUnit = Meter
            | Liter
            | KiloGram
              deriving (Show, Eq)

data ImperialUnit = Yard
              | Gallon
              | Pound
                deriving (Show, Eq)

data Measurement = MetricMeasurement Double MetricUnit
             | ImperialMeasurement Double ImperialUnit
               deriving (Show)

symbol :: MetricUnit -> String
symbol x
  | x == Meter = "m"
  | x == Liter = "L"
  | x == KiloGram = "kg"

convert (MetricMeasurement x u)
  | u==Meter    = ImperialMeasurement (1.0936*x) Yard
  | u==Liter    = ImperialMeasurement (0.2642*x) Gallon
  | u==KiloGram = ImperialMeasurement (2.2046*x) Pound

convert (ImperialMeasurement x u)
  | u==Yard   = MetricMeasurement (0.9144*x) Meter
  | u==Gallon = MetricMeasurement (3.7854*x) Liter
  | u==Pound  = MetricMeasurement (0.4536*x) KiloGram

let fac n = if n == 0 then 1 else n * fac (n-1)   --This line doesn't work
let m = MetricMeasurement 2 Meter

Answer 1

你不能let在顶层有语句。简单地写

fac n = if n == 0 then 1 else n * fac (n-1)
m = MetricMeasurement 2 Meter

Answer 2

删除let：

fac n = if n == 0 then 1 else n * fac (n-1)

m = MetricMeasurement 2 Meter

Answer 3

正如其他人指出的那样，您不let用于顶级定义，而仅用于函数内的本地定义。

我已经回答了，因为我忍不住指出您可以通过使用模式匹配来改进很多代码。

对于symbol，匹配可以在顶层进行：

symbol :: MetricUnit -> String
symbol Meter = "m"
symbol Liter = "L"
symbol KiloGram = "kg"

而对于其他两个，您可以使用 case 语句来匹配u，如下所示：

convert (MetricMeasurement x u) = case u of
   Meter    -> ImperialMeasurement (1.0936*x) Yard
   Liter    -> ImperialMeasurement (0.2642*x) Gallon
   KiloGram -> ImperialMeasurement (2.2046*x) Pound

并且convert (ImperialMeasurement x u)会相似。