Nested control structure in Haskell

Question

Is there any way to create nested control structure? For example I tried this. But I got error.

bmiTell :: (RealFloat a) => a -> String  

bmiTell bmi  = if bmi <= 18.5  then if bmi==16.0 then "asdasdasdsad"
           else if bmi <= 25.0 then "You're supposedly normal. Pffft, I bet  you're ugly!"  
           else if bmi <= 30.0 then "You're fat! Lose some weight, fatty!"  
           else    "You're a whale, congratulations!"

I think you need to add some curly braces to get that to work. — Pubby
– Pubby, Commented Mar 6, 2013 at 19:41
@pubby braces, while syntactically legal, are not required for or even very commonly seen in Haskell if-then-else statements. — Thomas M. DuBuisson
– Thomas M. DuBuisson, Commented Mar 6, 2013 at 20:25

dave4420 · Accepted Answer · 2013-03-06 20:00:22Z

12

The if expression is parsed as

if bmi <= 18.5   -- no else!
  then if bmi==16.0
         then "asdasdasdsad"
         else if bmi <= 25.0
                then "You're supposedly normal. Pffft, I bet  you're ugly!"  
                else if bmi <= 30.0
                       then "You're fat! Lose some weight, fatty!"  
                       else "You're a whale, congratulations!"

Note that the first if has a then branch but no else branch.

In Haskell, every if expression must have a then branch and an else branch. So that's your problem.

I second bchurchill's suggestion of using guards rather than nested if expressions.

edited Mar 6, 2013 at 20:00

answered Mar 6, 2013 at 19:44

dave4420

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2 Comments

molten Over a year ago

This is also gives error. The same:Syntax error in expression (unexpected `}', possibly due to bad layout)

dave4420 Over a year ago

@molten What do you want to happen if bmi is less than 18.5 but not equal to 16.0? That is basically the missing else branch.

dave4420 · Accepted Answer · 2013-03-06 20:01:10Z

9

Yeah, you just need to indent things properly. ghc probably doesn't like being told it's fat either. In either case, the indenting determines what branches correspond to what statements, and I might have messed up the order a bit:

bmiTell bmi  = if bmi <= 18.5  
               then if bmi==16.0 
                    then "asdasdasdsad"
                    else if bmi <= 25.0 
                         then "You're supposedly normal. Pffft, I bet  you're ugly!"  
                         else if bmi <= 30.0 
                              then "You're fat! Lose some weight, fatty!"  
                              else    "You're a whale, congratulations!"  
               else "foobar"

The better way to do this is with a guarded conditional, e.g.

bmiTell bmi
  | bmi < 18.5 = "foo"
  | bmi < 25.0 = "bar"
  | bmi < 30.0 = "buzz"

edited Mar 6, 2013 at 20:01

dave4420

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answered Mar 6, 2013 at 19:43

bchurchill

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4 Comments

molten Over a year ago

It gives this error:Syntax error in expression (unexpected `}', possibly due to bad layout). By the way I'm using hugs. How can I handle this?

molten Over a year ago

But the function with guards doesn't include nested structure.

dave4420 Over a year ago

@molten The function with guards had a mistake in it. Fixed now.

dave4420 Over a year ago

Indentation of if-then-else is irrelevant as far as the compiler is concerned (although of course it can make it more understandable for humans).

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