So I've only a little experience with Haskell and I've been working on the below program to implement a search to find maxima in a function, yet I've been receiving an odd error. When I compile it says:
MaximaSearch.hs:26:1: parse error (possibly incorrect indentation or mismatched brackets)
That's the line that says "main = do" so I think it's some sort of trailing error from my indentation in the code preceding it but I can't spot any mistakes...
Here's the code:
module Main where
g :: Float -> Float
--your function here
g x = cos(x^2)
--goldenSectionSearch
goldenSS :: (Float -> Float) -> Float -> Float -> Float -> Float -> Float
goldenSS f a b c tau
| (c-a) < tau * (abs b + abs x) = (c+a)/2
|f x > f b = let
t1|(c - b) > (b-a) = goldenSS f b x c tau
|otherwise = goldenSS f a x b tau
in t1
|otherwise = let
t2|(c-b) > (b-a) = goldenSS f a b x tau
|otherwise = goldenSS f x b c tau
in t2
where
let x
| (c-b) > (b-a) = b + resphi*(c-b)
|otherwise = b - resphi*(b-a)
where resphi = 2 - phi where phi = (1+ sqrt 5)/2
in x
--main
main = do
print x
print (g x)
where
x = goldenSS g a ((a+b)/2) b tau
where
a = 2
b = 3
tau = 0.001
any ideas?
The reason you're getting the parse error stems from the non-idiomatic usage of let and where bindings in your code.
Haskell allows multiple syntactic structures for temporary bindings and pattern matching, but you're combining them in a rather strange and messy manner.
To learn how to write the code cleaner and in a manner more usual for Haskell, I'd recommend looking up existing haskell libraries and programs (for example on hackage) to get a feel for how let and where bindings usually work. In general I find that for pure functions I almost exclusively use where (as opposed to let), but certain things are stylistic.
As for this code, I modified it a bit to use where bindings instead of let, and it compiles and runs for me now. Even if you have to tweak it a bit to get it to compile for you, this overall structure is cleaner and less likely to give you parse errors:
module Main where
g :: Float -> Float
g x = cos(x^2)
goldenSS :: (Float -> Float) -> Float -> Float -> Float -> Float -> Float
goldenSS f a b c tau
|(c-a) < tau * (abs b + abs x) = (c+a)/2
|f x > f b = t1
|otherwise = t2
where x | (c-b) > (b-a) = b + resphi*(c-b)
|otherwise = b - resphi*(b-a)
resphi = 2 - phi
phi = (1+ sqrt 5)/2
t1 |(c - b) > (b-a) = goldenSS f b x c tau
|otherwise = goldenSS f a x b tau
t2 |(c-b) > (b-a) = goldenSS f a b x tau
|otherwise = goldenSS f x b c tau
main =
do
print x
print (g x)
where x = goldenSS g a ((a+b)/2) b tau
a = 2
b = 3
tau = 0.001