diff --git a/doc/specs/stdlib_linalg.md b/doc/specs/stdlib_linalg.md
index 33fa43ed0..8f1fe2e47 100644
--- a/doc/specs/stdlib_linalg.md
+++ b/doc/specs/stdlib_linalg.md
@@ -187,6 +187,10 @@ to the original.
 
 Stable
 
+### Class
+
+Pure function.
+
 ### Description
 
 Create a diagonal array or extract the diagonal elements of an array
diff --git a/src/stdlib_linalg.fypp b/src/stdlib_linalg.fypp
index c56f40bed..c33668839 100644
--- a/src/stdlib_linalg.fypp
+++ b/src/stdlib_linalg.fypp
@@ -154,13 +154,13 @@ module stdlib_linalg
       ! Vector to matrix
       !
     #:for k1, t1 in RCI_KINDS_TYPES
-      module function diag_${t1[0]}$${k1}$(v) result(res)
+      module pure function diag_${t1[0]}$${k1}$(v) result(res)
         ${t1}$, intent(in) :: v(:)
         ${t1}$ :: res(size(v),size(v))
       end function diag_${t1[0]}$${k1}$
     #:endfor
     #:for k1, t1 in RCI_KINDS_TYPES
-      module function diag_${t1[0]}$${k1}$_k(v,k) result(res)
+      module pure function diag_${t1[0]}$${k1}$_k(v,k) result(res)
         ${t1}$, intent(in) :: v(:)
         integer, intent(in) :: k
         ${t1}$ :: res(size(v)+abs(k),size(v)+abs(k))
@@ -171,13 +171,13 @@ module stdlib_linalg
       ! Matrix to vector
       !
     #:for k1, t1 in RCI_KINDS_TYPES
-      module function diag_${t1[0]}$${k1}$_mat(A) result(res)
+      module pure function diag_${t1[0]}$${k1}$_mat(A) result(res)
         ${t1}$, intent(in) :: A(:,:)
         ${t1}$ :: res(minval(shape(A)))
       end function diag_${t1[0]}$${k1}$_mat
     #:endfor
     #:for k1, t1 in RCI_KINDS_TYPES
-      module function diag_${t1[0]}$${k1}$_mat_k(A,k) result(res)
+      module pure function diag_${t1[0]}$${k1}$_mat_k(A,k) result(res)
         ${t1}$, intent(in) :: A(:,:)
         integer, intent(in) :: k
         ${t1}$ :: res(minval(shape(A))-abs(k))
diff --git a/src/stdlib_linalg_diag.fypp b/src/stdlib_linalg_diag.fypp
index ec05abc42..3e74ab67f 100644
--- a/src/stdlib_linalg_diag.fypp
+++ b/src/stdlib_linalg_diag.fypp
@@ -7,7 +7,7 @@ submodule (stdlib_linalg) stdlib_linalg_diag
 contains
 
     #:for k1, t1 in RCI_KINDS_TYPES
-      module function diag_${t1[0]}$${k1}$(v) result(res)
+      module pure function diag_${t1[0]}$${k1}$(v) result(res)
         ${t1}$, intent(in) :: v(:)
         ${t1}$ :: res(size(v),size(v))
         integer :: i
@@ -20,7 +20,7 @@ contains
 
 
     #:for k1, t1 in RCI_KINDS_TYPES
-      module function diag_${t1[0]}$${k1}$_k(v,k) result(res)
+      module pure function diag_${t1[0]}$${k1}$_k(v,k) result(res)
         ${t1}$, intent(in) :: v(:)
         integer, intent(in) :: k
         ${t1}$ :: res(size(v)+abs(k),size(v)+abs(k))
@@ -44,7 +44,7 @@ contains
     #:endfor
 
     #:for k1, t1 in RCI_KINDS_TYPES
-      module function diag_${t1[0]}$${k1}$_mat(A) result(res)
+      module pure function diag_${t1[0]}$${k1}$_mat(A) result(res)
         ${t1}$, intent(in) :: A(:,:)
         ${t1}$ :: res(minval(shape(A)))
         integer :: i
@@ -55,7 +55,7 @@ contains
     #:endfor
 
     #:for k1, t1 in RCI_KINDS_TYPES
-      module function diag_${t1[0]}$${k1}$_mat_k(A,k) result(res)
+      module pure function diag_${t1[0]}$${k1}$_mat_k(A,k) result(res)
         ${t1}$, intent(in) :: A(:,:)
         integer, intent(in) :: k
         ${t1}$ :: res(minval(shape(A))-abs(k))