Why are Booleans bad?

Problem
Solution
type alias Request =
{ fetching : Bool
, error : String
, message : String
}
getResponse : Request -> ( String, Bool )
getResponse request =
if request.fetching then
( "", True)
else if error == "" then
( request.message, True )
else
( request.error, False )
type Request =
Fetching
| Error String
| Message String
getResponse : Request -> Maybe (Result String String)
getResponse request =
case request of
Fetching ->
Nothing
Error error ->
Just <| Err error
Ok message ->
Just <| Ok message

Question

I heard that Booleans should be avoided in Elm, How and Why?

Answer

Booleans create a lot of problems, for example what does it mean if Request.fetching is False but Request.message has some value? Or how can one know what the returned boolean of getResponse stands for? The type system of Elm can avoid such problems:

  • Use Maybe (String, Bool) instead of returning some default value.

  • Use Result String String to handle results

  • Use a Custom Type and Patter Matching instead of Request.fetching and If-Statements.

Further reading