[Haskell-cafe] Code layout in Emacs' haskell-mode

14 May 2007

      I am new to Haskell---and also to languages with the off-side
rule--and working my way through Hal Daume's tutorial. I'm a little
confused by the support for code layout in Emacs' haskell-mode. Is it
buggy, or am I doing something wrong.

For example, here's the "Hello, world" example from the tutorial, with
the indentation induced by pounding Tab in haskell-mode.

test.hs:
module Test
    where

      import IO

main = do
  putStrLn "Hello, world"

Prelude> :l test
[1 of 1] Compiling Test             ( test.hs, interpreted )

test.hs:12:0: parse error on input `main'

In emacs, every line but the one with "where" reports "Sole
indentation". With "where", I have the option of having it flush left
or indented four spaces; "import" wants to be two spaces in from
"where". Moving where doesn't change the error. But if I manually move
import flush left (which is the way it's shown in the tutorial, BTW):

module Test
    where

import IO

main = do
  putStrLn "Hello, world"

Prelude> :l test
[1 of 1] Compiling Test             ( test.hs, interpreted )
Ok, modules loaded: Test.

I have a similar problem with the layout of if-then-else...

num.hs:
module Num
    where

import IO

main = do
  putStrLn "Enter a number: "
  inp <- getLine
  let n = read inp
  if n == 0
  then putStrLn "Zero"
  else putStrLn "NotZero"

Prelude> :l num
[1 of 1] Compiling Num              ( num.hs, interpreted )

num.hs:11:2: parse error (possibly incorrect indentation)

Again, if I hit tab on the "then" or "else" lines, emacs reports "Sole
indentation". But if I manually change the indentation, it works.

module Num
    where

import IO

main = do
  putStrLn "Enter a number: "
  inp <- getLine
  let n = read inp
  if n == 0
     then putStrLn "Zero"
     else putStrLn "NotZero"

Prelude> :l num
[1 of 1] Compiling Num              ( num.hs, interpreted )
Ok, modules loaded: Num.

This is particularly weird because if-then-else doesn't always act this way:

exp.hs:
module Exp
    where

my_exponent a n =
    if n == 0
    then 1
    else a * my_exponent a (n-1)

Prelude> :l exp
[1 of 1] Compiling Exp              ( exp.hs, interpreted )
Ok, modules loaded: Exp.

I suppose this might have something to do with the do-notation...

Does haskell-mode support code layout? Are there conventions I need to
know about to make it behave properly? I have haskell-mode version
2.1-1 installed from the Ubuntu feisty repository.

Thanks,
Chris

[Haskell-cafe] Code layout in Emacs' haskell-mode

Christopher L Conway