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, 4 insertions, 4 deletions

diff --git a/compiler/typecheck/TcDerivInfer.hs b/compiler/typecheck/TcDerivInfer.hs
index 93dcf4383c..02c0103eec 100644
--- a/compiler/typecheck/TcDerivInfer.hs
+++ b/compiler/typecheck/TcDerivInfer.hs
@@ -67,12 +67,12 @@ inferConstraints :: [TyVar] -> Class -> [TcType] -> TcType
 inferConstraints tvs main_cls cls_tys inst_ty
                  rep_tc rep_tc_args
                  mechanism
-  | is_generic && not is_anyclass     -- Generic constraints are easy
+  | is_generic && not is_anyclass          -- Generic constraints are easy
   = return ([], tvs, inst_tys)
 
-  | is_generic1 && not is_anyclass    -- Generic1 needs Functor
-  = ASSERT( length rep_tc_tvs > 0 )   -- See Note [Getting base classes]
-    ASSERT( length cls_tys   == 1 )   -- Generic1 has a single kind variable
+  | is_generic1 && not is_anyclass         -- Generic1 needs Functor
+  = ASSERT( rep_tc_tvs `lengthExceeds` 0 ) -- See Note [Getting base classes]
+    ASSERT( cls_tys `lengthIs` 1 )         -- Generic1 has a single kind variable
     do { functorClass <- tcLookupClass functorClassName
        ; con_arg_constraints (get_gen1_constraints functorClass) }