diff --git a/src/biosequence/printing.jl b/src/biosequence/printing.jl
index a6dee497..eb6665bb 100644
--- a/src/biosequence/printing.jl
+++ b/src/biosequence/printing.jl
@@ -8,13 +8,13 @@ Base.summary(seq::BioSequence{<:AminoAcidAlphabet}) = string(length(seq), "aa ",
 # Buffer type. Not exposed to user, so code should be kept simple and performant.
 # B is true if it is buffered and false if it is not
 mutable struct SimpleBuffer{B, T} <: IO
+    const io::T
+    const arr::Vector{UInt8}
     len::UInt
-    arr::Vector{UInt8}
-    io::T
 end
 
-SimpleBuffer(io::IO) = SimpleBuffer{true, typeof(io)}(0, Vector{UInt8}(undef, 1024), io)
-SimpleBuffer(io::IO, len) = SimpleBuffer{false, typeof(io)}(0, Vector{UInt8}(undef, len), io)
+SimpleBuffer(io::IO) = SimpleBuffer{true, typeof(io)}(io, Vector{UInt8}(undef, 1024), 0)
+SimpleBuffer(io::IO, len) = SimpleBuffer{false, typeof(io)}(io, Vector{UInt8}(undef, len), 0)
 
 function Base.write(sb::SimpleBuffer{true}, byte::UInt8)
     sb.len ≥ 1024 && flush(sb)
diff --git a/src/longsequences/longsequence.jl b/src/longsequences/longsequence.jl
index b80d933b..e18f997b 100644
--- a/src/longsequences/longsequence.jl
+++ b/src/longsequences/longsequence.jl
@@ -84,7 +84,7 @@ The same applies with `LongSequence{RNAAlphabet{4}}`, simply replace the alphabe
 parameter with `RNAAlphabet{2}` in order to benefit.
 """
 mutable struct LongSequence{A <: Alphabet} <: BioSequence{A}
-    data::Vector{UInt64}  # encoded character sequence data
+    const data::Vector{UInt64}  # encoded character sequence data
     len::UInt
 
     function LongSequence{A}(data::Vector{UInt64}, len::UInt) where {A <: Alphabet}
diff --git a/src/search/re.jl b/src/search/re.jl
index 12521c55..ef17e3e7 100644
--- a/src/search/re.jl
+++ b/src/search/re.jl
@@ -51,8 +51,8 @@ import BioSequences
 # -----------
 
 mutable struct SyntaxTree
-    head::Symbol
-    args::Vector{Any}
+    const head::Symbol
+    const args::Vector{Any}
 
     function SyntaxTree(head, args)
         @assert head ∈ (
@@ -805,11 +805,11 @@ end
 
 # simple stack
 mutable struct Stack{T}
+    const data::Vector{T}
     top::Int
-    data::Vector{T}
 
     function Stack{T}(sz::Int=0) where T
-        return new{T}(0, Vector{T}(undef, sz))
+        return new{T}(Vector{T}(undef, sz), 0)
     end
 end