From cc6439ee19e843be1c86d8ee2eb69663316d1973 Mon Sep 17 00:00:00 2001
From: Xianda Sun <5433119+sunxd3@users.noreply.github.com>
Date: Sun, 27 Oct 2024 19:45:23 +0000
Subject: [PATCH] Add `fix` and `unfix` functions for parameter fixing and
 unfixing (#109)

* Add fix() and unfix() functions for parameter fixing and unfixing

* bump version and add export

* add to doc

* add sentence following Penny's comment
---
 Project.toml            |  2 +-
 docs/src/api.md         |  2 ++
 src/AbstractPPL.jl      |  2 +-
 src/abstractprobprog.jl | 41 +++++++++++++++++++++++++++++++++++++++++
 4 files changed, 45 insertions(+), 2 deletions(-)

diff --git a/Project.toml b/Project.toml
index 2a74776..db0655f 100644
--- a/Project.toml
+++ b/Project.toml
@@ -3,7 +3,7 @@ uuid = "7a57a42e-76ec-4ea3-a279-07e840d6d9cf"
 keywords = ["probablistic programming"]
 license = "MIT"
 desc = "Common interfaces for probabilistic programming"
-version = "0.9.0"
+version = "0.10.0"
 
 [deps]
 AbstractMCMC = "80f14c24-f653-4e6a-9b94-39d6b0f70001"
diff --git a/docs/src/api.md b/docs/src/api.md
index 0e7f3f8..e4ca9e2 100644
--- a/docs/src/api.md
+++ b/docs/src/api.md
@@ -29,6 +29,8 @@ string_to_varname
 AbstractProbabilisticProgram
 condition
 decondition
+fix
+unfix
 logdensityof
 AbstractContext
 evaluate!!
diff --git a/src/AbstractPPL.jl b/src/AbstractPPL.jl
index 33fa417..f121b97 100644
--- a/src/AbstractPPL.jl
+++ b/src/AbstractPPL.jl
@@ -18,7 +18,7 @@ export VarName,
 
 
 # Abstract model functions
-export AbstractProbabilisticProgram, condition, decondition, logdensityof, densityof, AbstractContext, evaluate!!
+export AbstractProbabilisticProgram, condition, decondition, fix, unfix, logdensityof, densityof, AbstractContext, evaluate!!
 
 # Abstract traces
 export AbstractModelTrace
diff --git a/src/abstractprobprog.jl b/src/abstractprobprog.jl
index 30b8b35..9051d3e 100644
--- a/src/abstractprobprog.jl
+++ b/src/abstractprobprog.jl
@@ -62,6 +62,47 @@ should hold for generative models `m` and arbitrary `obs`.
 """
 function condition end
 
+"""
+    fix(model, params)
+
+Fix the values of parameters specified in `params` within the probabilistic model `model`. 
+This operation is equivalent to treating the fixed parameters as being drawn from a point mass 
+distribution centered at the values specified in `params`. Thus these parameters no longer contribute
+to the accumulated log density. 
+
+Conceptually, this is similar to Pearl's do-operator in causal inference, where we intervene 
+on variables by setting them to specific values, effectively cutting off their dependencies 
+on their usual causes in the model.
+
+The invariant
+
+```
+m == unfix(fix(m, params))
+```
+
+should hold for any model `m` and parameters `params`.
+"""
+function fix end
+
+
+"""
+    unfix(model)
+
+Remove any fixed parameters from the model `model`, returning a new model without the fixed parameters.
+
+This function reverses the effect of `fix` by removing parameter constraints that were previously set.
+It returns a new model where all previously fixed parameters are allowed to vary according to their 
+original distributions in the model.
+
+The invariant
+
+```
+m == unfix(fix(m, params))
+```
+
+should hold for any model `m` and parameters `params`.
+"""
+function unfix end
 
 """
     rand([rng=Random.default_rng()], [T=NamedTuple], model::AbstractProbabilisticProgram) -> T