Include as little code as possible in the module trailer. This code is included verbatim in the generated parser, so if any of it can go in a separate module, do so.

Give type signatures for everything (see Section 2.4, “Type Signatures”. This is reported to improve things by about 50%. If there is a type signature for every single non-terminal in the grammar, then Happy automatically generates type signatures for most functions in the parser.

Simplify the grammar as much as possible (applies to everything, this one).

Use a recent version of GHC. Versions from 4.04 onwards have lower memory requirements for compiling Happy-generated parsers.

Using Happy's -g -a -c options when generating parsers to be compiled with GHC will help considerably.