From 9dcac8dd481b3099cf2fe0b73705248c8e74f5b2 Mon Sep 17 00:00:00 2001
From: Quinn Dougherty <quinnd@riseup.net>
Date: Thu, 30 Jun 2022 12:20:50 -0400
Subject: [PATCH] fixed \`-1\` I had forgotten; added basic mean tests

---
 .../__tests__/Distributions/Symbolic_test.res   | 17 +++++++++++++++++
 .../Distributions/SymbolicDist/SymbolicDist.res |  2 +-
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/packages/squiggle-lang/__tests__/Distributions/Symbolic_test.res b/packages/squiggle-lang/__tests__/Distributions/Symbolic_test.res
index 6ee9ffc8..152b580d 100644
--- a/packages/squiggle-lang/__tests__/Distributions/Symbolic_test.res
+++ b/packages/squiggle-lang/__tests__/Distributions/Symbolic_test.res
@@ -77,6 +77,23 @@ describe("(Symbolic) mean", () => {
     meanValue->unpackFloat->expect->ExpectJs.toBeFalsy
   })
 
+  testAll(
+    "of beta distributions from mean and standard dev",
+    list{(0.39, 0.1), (0.08, 0.1), (0.8, 0.3)},
+    tup => {
+      let (mean, stdev) = tup
+      let betaDistribution = SymbolicDist.Beta.fromMeanAndStdev(mean, stdev)
+      let meanValue =
+        betaDistribution->E.R2.fmap(d =>
+          run(FromDist(ToFloat(#Mean), d->DistributionTypes.Symbolic))
+        )
+      switch meanValue {
+      | Ok(value) => value->unpackFloat->expect->toBeCloseTo(mean)
+      | Error(err) => err->expect->toBe("shouldn't happen")
+      }
+    },
+  )
+
   testAll(
     "of lognormal distributions",
     list{(2.0, 4.0), (1e-7, 1e-2), (-1e6, 10.0), (1e3, -1e2), (-1e8, -1e4), (1e2, 1e-5)},
diff --git a/packages/squiggle-lang/src/rescript/Distributions/SymbolicDist/SymbolicDist.res b/packages/squiggle-lang/src/rescript/Distributions/SymbolicDist/SymbolicDist.res
index 61470d08..0b192b5f 100644
--- a/packages/squiggle-lang/src/rescript/Distributions/SymbolicDist/SymbolicDist.res
+++ b/packages/squiggle-lang/src/rescript/Distributions/SymbolicDist/SymbolicDist.res
@@ -135,7 +135,7 @@ module Beta = {
   let fromMeanAndStdev = (mean, stdev) => {
     // https://en.wikipedia.org/wiki/Beta_distribution#Mean_and_variance
     let var = stdev *. stdev
-    let sampleSize = mean *. (1.0 -. mean) /. var
+    let sampleSize = mean *. (1.0 -. mean) /. var -. 1.0
     mean->fromMeanAndSampleSize(sampleSize)
   }
 }