解析案例表达式时出错

Question

I am getting this compile error:

Files.hs:47:17: parse error on input ‘->’
Failed, modules loaded: none.

In the following section of code:

main :: IO ()
main = do
    args <- getArgs
    let f = case args of
        ("W":_) -> eoltoW
        --      ^  here's 47:17
        ("U":_) -> eoltoU
        _       -> fail "2 - 3 arguments required"
    case args of
        [_,i,o] -> editWith f i o
        [_,i]   -> catWith  f i
        [_]     -> fail "2 - 3 arguments required"

While I understand the logic could use some tidying up, I do not see where I am going wrong with case syntax. I figure it might be some weird interaction with do and let , but I can't find any clue as to how to correct it.

Note, I have ensured that I only use spaces for indentation

Edit:

It seems that adding a single space further of indentation (as below) is sufficient to prevent the error, but I am unclear as to why.

main = do
    args <- getArgs
    let f = case args of
         ("W":_) -> eoltoW
         --      ^  here's 47:17
         ("U":_) -> eoltoU
         _       -> fail "2 - 3 arguments required"

Answer 1

This is described in 2.7 and 10.3 . Basically, the rule for let … in a do block* is that all bindings have to be indented the same way:

let a = …
    b = …
    c = …

Furthermore, the "…" have to be more indented than the layout list. For example, the following is a syntax error:

let a =
    10
in a

In order to create a new layout-list, we need to indent it further (see note 1 in section 10.3), which is why

let a =
     10
in a

is completely fine. The same holds for your case . All of the cases have to be further indented than f due to the off-side rule.

^{* that rule actually holds** for more, ie for let , where , do and of .
** well, as long as you don't introduce additional braces}