Question

Why is it that when I do range in Haskell, this works:

[LT .. GT]

but this doesn’t:

[LT..GT]

and what does this cryptic error mean:

<interactive>:1:2:
    Failed to load interface for `LT':
      Use -v to see a list of the files searched for.

<interactive>:1:2:
    A section must be enclosed in parentheses thus: (`LT..` GT)

However, When I use Ints, the second form (without spaces) works:

[1..3]

Answer 1

It’s because LT.. is interpreted as the . operator in the LT module.

---

<interactive>:1:2:
    Failed to load interface for `LT':
      Use -v to see a list of the files searched for.

It means GHC cannot find a module named LT. The same message appears if you use a qualified name with a non-existing library:

Prelude> SDJKASD.sdfhj

<interactive>:1:1:
    Failed to load interface for `SDJKASD':
      Use -v to see a list of the files searched for.

---

<interactive>:1:2:
    A section must be enclosed in parentheses thus: (`LT..` GT)

In Haskell, a section is an infix operator with a partial application, e.g. (* 3), which is equivalent to \x -> x * 3.

In your case, LT.. is interpreted as an infix . operator, and the GT is part of the section formed with this operator.

A section must be enclosed in parenthesis, and since the misinterpretation does not, the parser will complain like this.

Another example of the error:

Prelude> [* 3]

<interactive>:1:2:
    A section must be enclosed in parentheses thus: (* 3)