拥抱不要让脚本文件使用关键字"let"

Question

我把这个文件作为一个模块加载到Hugs，但是得到这个错误:

文件:

错误。拥抱。hs:38 -输入语法错误(意外关键字"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

您不能在顶层使用let语句。只需写入

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，您不将let用于顶级定义，而仅用于函数中的局部定义。

我已经回答了，因为我忍不住要指出，你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

和convert (ImperialMeasurement x u)是相似的。