This is a thin haskell wrapper around the CMPH library, obtainable at http://cmph.sf.net. I assume you have it installed in /usr/local/lib. The motivation is mostly speed, and wren thornton's bytestring-trie seems to be the main competition: % ./dist/build/benchmark_trie/benchmark_trie * trie lookup: 1.136ns per iteration / 880403.98 per second. % ./dist/build/benchmark/benchmark * perfect lookup: 0.687ns per iteration / 1456455.69 per second. it also uses less space in the haskell heap, building once and doing the same number of lookups: PerfectHash: total alloc = 2,525,223,964 bytes (excludes profiling overheads) Trie: total alloc = 9,806,202,096 bytes (excludes profiling overheads) although this is not an entirely fair comparison given how much PerfectHash stores on the C side.