From beginner to beginner, they look to me more as syntax errors than indentation problem.

The syntax for let is

      let { decl ; ... decl }  in expression

 or in layout format

      let decl

           ...

           decl

      in expression

 

and your attempts do not respect this syntax. In all three cases, after  the keyword 'in' you need a full expression,

and neither a where clause nor a set of matches are a full expression by themselves. An expression is either a function apllication or a 'statement' like if or case, or maybe something more I can't recall.

 

In my haskell exercises I really appreciate the fact that the haskell mode of emacs has syntax hints (also

function hints for functions in the prelude). Even now that I know haskell basic syntax, I find it helpful.

 

Don't know of any other editors with the same capability for haskell, though, and if you are not an emacs user, learning

the not-so-standard ways of emacs might offset the benefits of having syntax hints.