This question already has several other perfectly good answers explaining how to solve your problem. I don’t want to do that; instead, I will go through each line of your code, progressively correct the problems, and hopefully help you understand Haskell a bit better.
First, I’ll copy your code for convenience:
allEqual :: Eq a => a -> a -> a -> Bool
allEqual x y z do
Bool check <- x == y
Bool nextC <- y == z
if check == nextC
then True
else False
The first line is the type signature; this is already explained well in other answers, so I’ll skip this and go on to the next line.
The second line is where you are defining your function. The first thing you’ve missed is that you need an equals sign to define a function: function definition syntax is functionName arg1 arg2 arg3 … = functionBody, and you can’t remove the =. So let’s correct that:
allEqual :: Eq a => a -> a -> a -> Bool
allEqual x y z = do
Bool check <- x == y
Bool nextC <- y == z
if check == nextC
then True
else False
The next error is using do notation. do notation is notorious for confusing beginners, so don’t feel bad about misusing it. In Haskell, do notation is only used in specific situations where it is necessary to execute a sequence of statements line by line, and especially when you have some side-effect (like, say, printing to the console) which is executed with each line. Clearly, this doesn’t fit here — all you’re doing is comparing some values and returning a result, which is hardly something which requires line-by-line execution. So let’s get rid of that do:
allEqual :: Eq a => a -> a -> a -> Bool
allEqual x y z =
let Bool check = x == y
Bool nextC = y == z
in
if check == nextC
then True
else False
(I’ve also replaced the <- binding with let … in …, since <- can only be used within a do block.)
Next, another problem: Bool check is not valid Haskell! You may be familiar with this syntax from other languages, but in Haskell, a type is always specified using ::, and often with a type signature. So I’ll remove Bool before the names and add type signatures instead:
allEqual :: Eq a => a -> a -> a -> Bool
allEqual x y z =
let check :: Bool
check = x == y
nextC :: Bool
nextC = y == z
in
if check == nextC
then True
else False
Now, at this point, your program is perfectly valid Haskell — you’ll be able to compile it, and it will work. But there’s still a few improvements you can make.
For a start, you don’t need to include types — Haskell has type inference, and in most cases it’s fine to leave types out (although it’s traditional to include them for functions). So let’s get rid of the types in the let:
allEqual :: Eq a => a -> a -> a -> Bool
allEqual x y z =
let check = x == y
nextC = y == z
in
if check == nextC
then True
else False
Now, check and nextC are only used in one place — giving them names doesn’t do anything, and only serves to make the code less readable. So I’ll inline the definitions of check and nextC into their usages:
allEqual :: Eq a => a -> a -> a -> Bool
allEqual x y z =
if (x == y) == (y == z)
then True
else False
Finally, I see you have an expression of the form if <condition> then True else False. This is redundant — you can simply return the <condition> with the same meaning. So let’s do that:
allEqual :: Eq a => a -> a -> a -> Bool
allEqual x y z = (x == y) == (y == z)
This is much, much better than the code you started with!
(There is actually one more improvement that you can make to this code. At this point, it should be obvious that your code has a bug. Can you find it? And if so, can you fix it? Hint: you can use the && operator to ‘and’ two booleans together.)
