In Haskell, where does the range ['a'..] stop?

Question

I'm learning Haskell for fun, following Learn you a Haskell. Range can go to infinity with numbers when using [1..]. With characters, where does the range ['a'..] stop ? I kinda suppose the last character of the Unicode table, but I really know nothing about Haskell so just asking !

Answer 1

If you want to know where the range stops, just get the last item…

ghci> last ['a'.. ] '\1114111' 

Another method: Char is a bounded type, i.e. a type with minimum and maximum values defined. Types in the Bounded class provide a maxBound value. Since maxBound is polymorphic, you have to explicitly state the type of the value you're looking for.

ghci> maxBound :: Char '\1114111' ghci> maxBound :: Int 9223372036854775807 

There is no maxBound for Integer since they are unbounded.

The Haskell prelude explains the connection between ['a'..] and maxBound. The notation ['a'..] is syntactic sugar for enumFrom a; here enumFrom is a method of the Enum type class. The documentation of Enum specifies that when the type is also an instance of Bounded, enumFrom x should be equivalent to enumFromTo x maxBound, or more readably, [x..] = [x..maxBound]. So the last element of ['a'..] must be the same as maxBound :: Char.

last ['a'.. ] does iterate over the list, but a modern computer can count to a million in an eyeblink. It's not something you would want to put in a tight loop, but as something you run just once it isn't a big burden. If you try it out in a less optimized implementation such as Hugs instead of GHC, you may need to wait for the result. In contrast last [0..] would take approximately forever.