In Haskell, when do we use in with let?

Question

In the following code, the last phrase I can put an in in front. Will it change anything?

Another question: If I decide to put in in front of the last phrase, do I need to indent it?

I tried without indenting and hugs complains

Last generator in do {...} must be an expression

import Data.Char
groupsOf _ [] = []
groupsOf n xs = 
    take n xs : groupsOf n ( tail xs )

problem_8 x = maximum . map product . groupsOf 5 $ x
main = do t <- readFile "p8.log" 
          let digits = map digitToInt $concat $ lines t
          print $ problem_8 digits

---

Edit

Ok, so people don't seem to understand what I'm saying. Let me rephrase: are the following two the same, given the context above?

1.

let digits = map digitToInt $concat $ lines t
print $ problem_8 digits

2.

let digits = map digitToInt $concat $ lines t
in print $ problem_8 digits

Another question concerning the scope of bindings declared in let: I read here that:

where Clauses.

Sometimes it is convenient to scope bindings over several guarded equations, which requires a where clause:

f x y  |  y>z           =  ...
       |  y==z          =  ...
       |  y<z           =  ...
     where z = x*x

Note that this cannot be done with a let expression, which only scopes over the expression which it encloses.

My question: so, the variable digits shouldn't be visible to the last print phrase. Do I miss something here?

Answer 1

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.