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, 2 insertions, 1 deletions

diff --git a/compiler/main/InteractiveEval.hs b/compiler/main/InteractiveEval.hs
index b56d8d61e0..8b5a6b6af7 100644
--- a/compiler/main/InteractiveEval.hs
+++ b/compiler/main/InteractiveEval.hs
@@ -80,6 +80,7 @@ import RtClosureInspect
 import Outputable
 import FastString
 import Bag
+import Util
 import qualified Lexer (P (..), ParseResult(..), unP, mkPState)
 import qualified Parser (parseStmt, parseModule, parseDeclaration, parseImport)
 
@@ -400,7 +401,7 @@ moveHist fn = do
             history = resumeHistory r
             new_ix = fn ix
         --
-        when (new_ix > length history) $ liftIO $
+        when (history `lengthLessThan` new_ix) $ liftIO $
            throwGhcExceptionIO (ProgramError "no more logged breakpoints")
         when (new_ix < 0) $ liftIO $
            throwGhcExceptionIO (ProgramError "already at the beginning of the history")