Use lengthIs and friends in more places

While investigating #12545, I discovered several places in the code
that performed length-checks like so:

```
length ts == 4
```

This is not ideal, since the length of `ts` could be much longer than 4,
and we'd be doing way more work than necessary! There are already a slew
of helper functions in `Util` such as `lengthIs` that are designed to do
this efficiently, so I found every place where they ought to be used and
did just that. I also defined a couple more utility functions for list
length that were common patterns (e.g., `ltLength`).

Test Plan: ./validate

Reviewers: austin, hvr, goldfire, bgamari, simonmar

Reviewed By: bgamari, simonmar

Subscribers: goldfire, rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3622

1 files changed, 3 insertions, 2 deletions

diff --git a/ghc/GHCi/UI/Monad.hs b/ghc/GHCi/UI/Monad.hs
index b57a5a0a64..bb946cc7b1 100644
--- a/ghc/GHCi/UI/Monad.hs
+++ b/ghc/GHCi/UI/Monad.hs
@@ -46,6 +46,7 @@ import GHCi
 import GHCi.RemoteTypes
 import HsSyn (ImportDecl)
 import RdrName (RdrName)
+import Util
 
 import Exception
 import Numeric
@@ -396,8 +397,8 @@ printTimes dflags mallocs secs
   where
     separateThousands n = reverse . sep . reverse . show $ n
       where sep n'
-              | length n' <= 3 = n'
-              | otherwise = take 3 n' ++ "," ++ sep (drop 3 n')
+              | n' `lengthAtMost` 3 = n'
+              | otherwise           = take 3 n' ++ "," ++ sep (drop 3 n')
 
 -----------------------------------------------------------------------------
 -- reverting CAFs