diff --git a/Cargo.lock b/Cargo.lock
index 629461244d..24d9f53a1c 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -10,7 +10,7 @@ dependencies = [
 
 [[package]]
 name = "ascii"
-version = "0.9.1"
+version = "0.9.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
@@ -20,7 +20,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "byteorder"
-version = "1.3.1"
+version = "1.3.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
@@ -33,10 +33,10 @@ name = "combine"
 version = "3.8.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
- "ascii 0.9.1 (registry+https://github.com/rust-lang/crates.io-index)",
- "byteorder 1.3.1 (registry+https://github.com/rust-lang/crates.io-index)",
- "either 1.5.1 (registry+https://github.com/rust-lang/crates.io-index)",
- "memchr 2.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "ascii 0.9.2 (registry+https://github.com/rust-lang/crates.io-index)",
+ "byteorder 1.3.2 (registry+https://github.com/rust-lang/crates.io-index)",
+ "either 1.5.2 (registry+https://github.com/rust-lang/crates.io-index)",
+ "memchr 2.2.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "unreachable 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
@@ -52,7 +52,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "either"
-version = "1.5.1"
+version = "1.5.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
@@ -69,6 +69,14 @@ dependencies = [
  "num 0.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
+[[package]]
+name = "fxhash"
+version = "0.2.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "byteorder 1.3.2 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
 [[package]]
 name = "im-rc"
 version = "13.0.0"
@@ -99,7 +107,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "memchr"
-version = "2.2.0"
+version = "2.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
@@ -201,6 +209,7 @@ dependencies = [
  "combine 3.8.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "dogged 0.2.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "fraction 0.6.2 (registry+https://github.com/rust-lang/crates.io-index)",
+ "fxhash 0.2.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "im-rc 13.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "log 0.4.6 (registry+https://github.com/rust-lang/crates.io-index)",
  "maplit 1.0.1 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -277,21 +286,22 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [metadata]
 "checksum ansi_term 0.11.0 (registry+https://github.com/rust-lang/crates.io-index)" = "ee49baf6cb617b853aa8d93bf420db2383fab46d314482ca2803b40d5fde979b"
-"checksum ascii 0.9.1 (registry+https://github.com/rust-lang/crates.io-index)" = "a5fc969a8ce2c9c0c4b0429bb8431544f6658283c8326ba5ff8c762b75369335"
+"checksum ascii 0.9.2 (registry+https://github.com/rust-lang/crates.io-index)" = "91e320562a8fa3286a481b7189f89578ace6b20df99e123c87f2f509c957c5d6"
 "checksum autocfg 0.1.4 (registry+https://github.com/rust-lang/crates.io-index)" = "0e49efa51329a5fd37e7c79db4621af617cd4e3e5bc224939808d076077077bf"
-"checksum byteorder 1.3.1 (registry+https://github.com/rust-lang/crates.io-index)" = "a019b10a2a7cdeb292db131fc8113e57ea2a908f6e7894b0c3c671893b65dbeb"
+"checksum byteorder 1.3.2 (registry+https://github.com/rust-lang/crates.io-index)" = "a7c3dd8985a7111efc5c80b44e23ecdd8c007de8ade3b96595387e812b957cf5"
 "checksum cfg-if 0.1.6 (registry+https://github.com/rust-lang/crates.io-index)" = "082bb9b28e00d3c9d39cc03e64ce4cea0f1bb9b3fde493f0cbc008472d22bdf4"
 "checksum combine 3.8.1 (registry+https://github.com/rust-lang/crates.io-index)" = "da3da6baa321ec19e1cc41d31bf599f00c783d0517095cdaf0332e3fe8d20680"
 "checksum difference 2.0.0 (registry+https://github.com/rust-lang/crates.io-index)" = "524cbf6897b527295dff137cec09ecf3a05f4fddffd7dfcd1585403449e74198"
 "checksum dogged 0.2.0 (registry+https://github.com/rust-lang/crates.io-index)" = "2638df109789fe360f0d9998c5438dd19a36678aaf845e46f285b688b1a1657a"
-"checksum either 1.5.1 (registry+https://github.com/rust-lang/crates.io-index)" = "c67353c641dc847124ea1902d69bd753dee9bb3beff9aa3662ecf86c971d1fac"
+"checksum either 1.5.2 (registry+https://github.com/rust-lang/crates.io-index)" = "5527cfe0d098f36e3f8839852688e63c8fff1c90b2b405aef730615f9a7bcf7b"
 "checksum fixedbitset 0.1.9 (registry+https://github.com/rust-lang/crates.io-index)" = "86d4de0081402f5e88cdac65c8dcdcc73118c1a7a465e2a05f0da05843a8ea33"
 "checksum fraction 0.6.2 (registry+https://github.com/rust-lang/crates.io-index)" = "1055159ac82fb210c813303f716b6c8db57ace9d5ec2dbbc2e1d7a864c1dd74e"
+"checksum fxhash 0.2.1 (registry+https://github.com/rust-lang/crates.io-index)" = "c31b6d751ae2c7f11320402d34e41349dd1016f8d5d45e48c4312bc8625af50c"
 "checksum im-rc 13.0.0 (registry+https://github.com/rust-lang/crates.io-index)" = "0a0197597d095c0d11107975d3175173f810ee572c2501ff4de64f4f3f119806"
 "checksum lazy_static 1.3.0 (registry+https://github.com/rust-lang/crates.io-index)" = "bc5729f27f159ddd61f4df6228e827e86643d4d3e7c32183cb30a1c08f604a14"
 "checksum log 0.4.6 (registry+https://github.com/rust-lang/crates.io-index)" = "c84ec4b527950aa83a329754b01dbe3f58361d1c5efacd1f6d68c494d08a17c6"
 "checksum maplit 1.0.1 (registry+https://github.com/rust-lang/crates.io-index)" = "08cbb6b4fef96b6d77bfc40ec491b1690c779e77b05cd9f07f787ed376fd4c43"
-"checksum memchr 2.2.0 (registry+https://github.com/rust-lang/crates.io-index)" = "2efc7bc57c883d4a4d6e3246905283d8dae951bb3bd32f49d6ef297f546e1c39"
+"checksum memchr 2.2.1 (registry+https://github.com/rust-lang/crates.io-index)" = "88579771288728879b57485cc7d6b07d648c9f0141eb955f8ab7f9d45394468e"
 "checksum num 0.2.0 (registry+https://github.com/rust-lang/crates.io-index)" = "cf4825417e1e1406b3782a8ce92f4d53f26ec055e3622e1881ca8e9f5f9e08db"
 "checksum num-bigint 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)" = "57450397855d951f1a41305e54851b1a7b8f5d2e349543a02a2effe25459f718"
 "checksum num-complex 0.2.3 (registry+https://github.com/rust-lang/crates.io-index)" = "fcb0cf31fb3ff77e6d2a6ebd6800df7fdcd106f2ad89113c9130bcd07f93dffc"
diff --git a/Cargo.toml b/Cargo.toml
index 1040a3c72d..0d630ef455 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -11,6 +11,7 @@ combine = "3.8.1"
 im-rc = "13.0.0"
 fraction = "0.6.2"
 num = "0.2.0"
+fxhash = "0.2.1"
 
 [dev-dependencies]
 pretty_assertions = "0.5.1"
diff --git a/src/collections.rs b/src/collections.rs
new file mode 100644
index 0000000000..34760bb466
--- /dev/null
+++ b/src/collections.rs
@@ -0,0 +1,21 @@
+use std::hash::BuildHasherDefault;
+
+pub use fxhash::FxHasher;
+
+// Versions of HashMap and HashSet from both std and im_rc
+// which use the FNV hasher instead of the default SipHash hasher.
+// FNV is faster but less secure; that's fine, since this compiler
+// doesn't need cryptographically secure hashes, and also is not a
+// server concerned about hash flooding attacks!
+
+pub type MutMap<K, V> =
+    std::collections::HashMap<K, V, BuildHasherDefault<FxHasher>>;
+
+pub type MutSet<K> =
+    std::collections::HashSet<K, BuildHasherDefault<FxHasher>>;
+
+pub type ImMap<K, V> =
+    im_rc::hashmap::HashMap<K, V, BuildHasherDefault<FxHasher>>;
+
+pub type ImSet<K> =
+    im_rc::hashset::HashSet<K, BuildHasherDefault<FxHasher>>;
diff --git a/src/lib.rs b/src/lib.rs
index 6389abc54a..c94aaf905f 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -6,6 +6,7 @@ pub mod operator;
 pub mod region;
 pub mod fast_fraction;
 pub mod stdlib;
+mod collections;
 // mod ena;
 
 // #[macro_use]
@@ -17,5 +18,6 @@ extern crate dogged;
 extern crate im_rc;
 extern crate fraction;
 extern crate num;
+extern crate fxhash;
 
 #[macro_use] extern crate combine;