Fix Matern Grads (JuliaGaussianProcesses#478)

* gitignore dev folder

* Fix up matern

* Bump patch

* Avoid promotion with irrational

Author: willtebbutt

.gitignore

@@ -3,3 +3,4 @@
 Manifest.toml
 coverage/
 .DS_store
+dev

Project.toml

@@ -1,6 +1,6 @@
 name = "KernelFunctions"
 uuid = "ec8451be-7e33-11e9-00cf-bbf324bd1392"
-version = "0.10.45"
+version = "0.10.46"
 
 [deps]
 ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"

src/basekernels/matern.jl

@@ -37,14 +37,16 @@ MaternKernel(; nu::Real=1.5, ν::Real=nu, metric=Euclidean()) = MaternKernel(ν,
 
 @functor MaternKernel
 
-@inline function kappa(k::MaternKernel, d::Real)
-    result = _matern(only(k.ν), d)
-    return ifelse(iszero(d), one(result), result)
-end
+@inline kappa(k::MaternKernel, d::Real) = _matern(only(k.ν), d)
 
 function _matern(ν::Real, d::Real)
-    y = sqrt(2ν) * d
-    return exp((one(d) - ν) * logtwo - loggamma(ν) + ν * log(y) + log(besselk(ν, y)))
+    if iszero(d)
+        return one(d)
+    else
+        y = sqrt(2ν) * d
+        b = log(besselk(ν, y))
+        return exp((one(d) - ν) * oftype(y, logtwo) - loggamma(ν) + ν * log(y) + b)
+    end
 end
 
 metric(k::MaternKernel) = k.metric