TemplateHaskell not installed... then it is

Question

I'm writing a program in Haskell, and it involves a lot of parentheses. So to clear up that ugly mess, I use the $ operator a couple of times to make it easier to read. For example:

longFunc arg1 (anotherFunc (yetAnotherFunc arg2))

is replaced by

longFunc arg1 $ anotherFunc $ yetAnotherFunc arg2

But when I compile my program using GHCi I get a message saying:

MyFile.hs:18:18: error:
    parse error on input ‘$’
    Perhaps you intended to use TemplateHaskell
Failed, modules loaded: none.

Here's lines 16-18:

isDigit :: Char -> Bool
isDigit c =
  c `elem` $ ['0'..'9'] ++ "."

I was confused because I have used the $ operator several times (with the same compiler) like this:

main = putStrLn $ show 42

So I typed that code into my file as a test, deleted the other $ occurrences, and loaded it up.

And it worked!

Can someone please tell me what's going on?

Line 18 is pretty much the same thing as my example with the functions that are called with `$` instead of parentheses — InfiniteDonuts, Apr 15 '20 at 13:01
But the error is on line 18, and not somewhere else. _Something_ is clearly different about that line. There are places where `($)` is not allowed, or may not do what you expect, e.g. on the left of an assignment. — A. R., Apr 15 '20 at 13:03

score 7 · Accepted Answer · answered Apr 15 '20 at 13:15

You can't use ($) immediately after another infix operator. Line 18:

c `elem` $ ['0'..'9'] ++ "."

needs to be rewritten as one of the following options:

Keep the parentheses as original:
```
c `elem` (['0'..'9'] ++ "."])
```
Apply ($) to a slice of elem:
```
(c `elem`) $ ['0'..'9'] ++ "."
```
Convert the call to elem to a prefix call:
```
elem c $ ['0'..'9'] ++ "."
```

I recommend option 3. Consecutive infix operators (in this case `elem` and $) do not have a well-defined priority and confuse the parser. The general rule of thumb is that an infix operator must always have a complete expression on each side. c `elem` is not a complete expression, so it is not allowed on the left of $. Similarly, $ ['0'..'9'] ++ "." is not a complete expression, so it is not allowed to be the right side of `elem`.

Thanks! That really helped! – InfiniteDonuts Apr 15 '20 at 13:41 — InfiniteDonuts, Apr 15 '20 at 13:41

TemplateHaskell not installed... then it is

1 Answers1