problem with let in binding in a predicate in haskell language?

Question

i'm newbie in haskell language. i wrote function get list of pairs as input and return list of BMI index:

calcBMI:: (RealFloat a)=> [(a,a)]->[a]

calcBMI xs = [ result | (w,h)<-xs, let bmifunc (w,h)= w/h^2; result =bmifunc (w,h) in result >=25]

when i save and :reload in ghci, error: Not in scope: `result' Failed, modules loaded: none appear. i think the list comprehension don't know what is resultwhich i have introduced in predicate of list comprehension. Please tell me why? and how to fix the problem

daylily daylily · Accepted Answer · 2021-07-13T13:59:57

Any variables defined in let ... in is only effective in the expression after in. So, you cannot use your result in anywhere else. Luckily Haskell list comprehension expression allows you to define variables in it that is referrable anywhere in the expression, by simply adding a let x = ... generator. Hence instead of writing

calcBMI xs =
  [result | (w, h) <- xs,
            let bmifunc (w, h) = w / h ^ 2;
                result = bmifunc (w, h)
             in result >= 25]

You can write

calcBMI xs =
  [result | (w, h) <- xs,
            let bmifunc (w, h) = w / h ^ 2;
                result = bmifunc (w, h),
            result >= 25]

and the code will compile.

problem with let in binding in a predicate in haskell language?

1 Answers