[Haskell-cafe] Re: Indentation Question.

[mail at justinbogner.com]

"Ramaswamy, Vivek" <Vivek.Ramaswamy at FMR.COM> writes:
> Hello All~
>
> I have been reading the book ?Haskell for real programmers? and am presently on
> chapter 03.
>
> There was a small program demonstrated by the author on ?let?. The problem is
> that when I cut and paste authors code into eclipse, the program works fine,
> where as when I write my own copy of the program I am getting an error. I
> figured out that when I remove the function any one function lend or bend, the
> file compiles and runs fine.
>
> My question is why does the Haskell compiler complains, in the case 2 functions
> with the same signature and logic, even though their names are different.
>
> module Lending where
>
> {-- snippet lend --}
>
> lend amount balance = let reserve    = 100
>
>                           newBalance = balance - amount
>
>                       in if balance < reserve
>
>                          then Nothing
>
>                          else Just newBalance
>
> {-- /snippet lend --}
>
> bend amount balance = let reserver = 100
>
>                                   newBalance=balance-amount

The problem is here                 ^^^^

each element in a let clause should be indented to the same level, that
is

  let foo = bar
      baz = qux

is legal. foo and baz are both defined. But

  let foo = bar
       baz = qux

is not legal, the compiler thinks baz = qux is part of the statement
`foo = bar`, like `foo = bar baz = qux`. Also

  let foo = bar
     baz = qux

is not legal, since the compiler thinks the let clause is over and
expects the keyword `in`

>
>                             in if balance < reserver
>
>                                  then Nothing
>
>                                  else Just newBalance
>
> Regards
>
> -Vivek Ramaswamy-