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

2 files changed, 3 insertions, 2 deletions

diff --git a/compiler/vectorise/Vectorise/Type/Env.hs b/compiler/vectorise/Vectorise/Type/Env.hs
index 612c051a2c..9526feddaf 100644
--- a/compiler/vectorise/Vectorise/Type/Env.hs
+++ b/compiler/vectorise/Vectorise/Type/Env.hs
@@ -365,7 +365,7 @@ vectTypeEnv tycons vectTypeDecls vectClassDecls
         defDataCons
           | isAbstract = return ()
           | otherwise
-          = do { MASSERT(length (tyConDataCons origTyCon) == length (tyConDataCons vectTyCon))
+          = do { MASSERT(tyConDataCons origTyCon `equalLength` tyConDataCons vectTyCon)
                ; zipWithM_ defDataCon (tyConDataCons origTyCon) (tyConDataCons vectTyCon)
                }
 
diff --git a/compiler/vectorise/Vectorise/Utils/PADict.hs b/compiler/vectorise/Vectorise/Utils/PADict.hs
index 9cd740cf53..4d32f5df74 100644
--- a/compiler/vectorise/Vectorise/Utils/PADict.hs
+++ b/compiler/vectorise/Vectorise/Utils/PADict.hs
@@ -22,6 +22,7 @@ import Var
 import Outputable
 import DynFlags
 import FastString
+import Util
 import Control.Monad
 
 
@@ -199,7 +200,7 @@ prDFunApply dfun tys
   = return $ Var dfun `mkTyApps` tys
 
   | Just tycons <- ctxs
-  , length tycons == length tys
+  , tycons `equalLength` tys
   = do
       pa <- builtin paTyCon
       pr <- builtin prTyCon