In Haskell, when do we use in with let?

Short answer: Use let without in in the body of a do-block, and in the part after the | in a list comprehension. Anywhere else, use let ... in ....

The keyword let is used in three ways in Haskell.

1. The first form is a let-expression.

   let variable = expression in expression
   

   This can be used wherever an expression is allowed, e.g.

   > (let x = 2 in x*2) + 3
   7
   

2. The second is a let-statement. This form is only used inside of do-notation, and does not use in.

   do statements
      let variable = expression
      statements
   

3. The third is similar to number 2 and is used inside of list comprehensions. Again, no in.

   > [(x, y) | x <- [1..3], let y = 2*x]
   [(1,2),(2,4),(3,6)]
   

   This form binds a variable which is in scope in subsequent generators and in the expression before the |.

The reason for your confusion here is that expressions (of the correct type) can be used as statements within a do-block, and let .. in .. is just an expression.

Because of the indentation rules of haskell, a line indented further than the previous one means it’s a continuation of the previous line, so this

do let x = 42 in
     foo

gets parsed as

do (let x = 42 in foo)

Without indentation, you get a parse error:

do (let x = 42 in)
   foo

In conclusion, never use in in a list comprehension or a do-block. It is unneccesary and confusing, as those constructs already have their own form of let.