我正在学习Haskell，并正在从书中编写示例程序。下面的模块无法编译，并在ghci和ghc中给我以下错误：
“ Optimal.hs：15：23：输入'='时解析错误”
有什么想法，我看不到吗？
module Optimal (optimalPath) where
import RoadSystem
optimalPath :: RoadSystem -> Path
optimalPath roadSystem =
let (bestAPath, bestBPath) = foldl roadStep ([], []) roadSystem
in if sum (map snd bestAPath) <= sum (map snd bestBPath)
then reverse bestAPath
else reverse bestBPath
roadStep :: (Path, Path) -> Section -> (Path, Path)
roadStep (pathA, pathB) (Section a b c) =
let timeA = sum (map snd pathA)
timeB = sum (map snd pathB)
forwardTimeToA = timeA + a
crossTimeToA = timeB + b + c
forwardTimeToB = timeB + b
crossTimeToB = timeA + a + c
newPathToA = if forwardTimeToA <= crossTimeToA
then (A, a):pathA
else (C, c):(B, b):pathB
newPathToB = if forwardTimeToB <= crossTimeToB
then (B, b):pathB
else (C, c):(A, a):pathA
in (newPathToA, newPathToB)
有什么想法，我看不到吗？
当您使用制表符进行缩进时，会发生这种情况。该哈斯克尔标准任务承担的制表位8个字符，所以编译器实际上是认为你的代码是：
module Optimal (optimalPath) where
import RoadSystem
optimalPath :: RoadSystem -> Path
optimalPath roadSystem =
let (bestAPath, bestBPath) = foldl roadStep ([], []) roadSystem
in if sum (map snd bestAPath) <= sum (map snd bestBPath)
then reverse bestAPath
else reverse bestBPath
roadStep :: (Path, Path) -> Section -> (Path, Path)
roadStep (pathA, pathB) (Section a b c) =
let timeA = sum (map snd pathA)
timeB = sum (map snd pathB)
forwardTimeToA = timeA + a
crossTimeToA = timeB + b + c
forwardTimeToB = timeB + b
crossTimeToB = timeA + a + c
newPathToA = if forwardTimeToA <= crossTimeToA
then (A, a):pathA
else (C, c):(B, b):pathB
newPathToB = if forwardTimeToB <= crossTimeToB
then (B, b):pathB
else (C, c):(A, a):pathA
in (newPathToA, newPathToB)
并且在第23行第23行处有足够的把握：
let timeA = sum (map snd pathA)
timeB = sum (map snd pathB)
这是语法错误。
要解决这个问题：
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我来说两句