haskell compile parse error (possibly incorrect indentation)-CodePudding

i am new to haskell and i am trying to make the following code work:

abc :: fooType
abc = do
    let loop c d = do 
      let q = borrow "a"
          d2 = d   q
          c2 = c   1
          if c2 == 10
            then 
              if d2 >= 60
                then
                  maybeB <- cast "b"
                  return $ isJust maybeB 
                else
                  return $ False
          else
            loop c2 d2
    
    loop 0 0

i keep getting:

error: parse error on input `let'
let q = borrow "a"

the code looks correct to me, would that be a problem with indentation? I know do blocks have some specific rules on how to set up indentation but from what i've been reading it looks correct.., can anyone spot what the problem is?

ps: borrow returns Int > 0, cast returns maybe int. But the compilation is failing on line2

CodePudding user response：

let and do are both block heralds -- that means that the start of the next lexical token starts a block and sets the indentation level of that block. Additionally, indentation must increase to nest blocks. So:

abc = do -- block herald
  -- "let" is the next token after a block herald, so it sets an indentation level;
  --   it is also itself a block herald
  -- "loop" is the next token after a block herald, so it sets an indentation level
  -- "do" is a block herald
  let loop = do
        -- "let" is the next token after a block herald, so it sets an indentation level,
        --   **and must be deeper than the enclosing indentation;**
        --   it is also a block herald
        -- "q" is the next token after a block herald, so it sets an indentation level
        let q = ...
            -- "d2" must use the indentation level of the block it wants to be in;
            --   presumably the enclosing "let"
            d2 = ...
            c2 = ...
        -- this "if" must use the indentation of the block it wants to be in;
        --   presumably the closest enclosing "do" block
        if ...
        -- this "then" can be indented the same as the "if" above it, but I find
        --   that confusing; my preferred style indicates the nesting structure
          then ...
          else ...
  -- this "loop" must use the indentation of the block it wants to be in; presumably the outermost "do" block
  loop

There's a lot going on in the second line, so it's easy to lose track of all the things you need to think about. Anyway, the main mistake in your code is that loop sets an indentation level, and the let shortly after it must be indented more than it.