# Type stability of different for loops

**URL:** <https://discourse.julialang.org/t/type-stability-of-different-for-loops/17431>\
**Category:** General Usage\
**Tags:** question\
**Created:** [November 12, 2018, 1:07pm UTC](https://discourse.julialang.org/t/type-stability-of-different-for-loops/17431 "2018-11-12T13:07:25Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Christopher\_Fisher](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/christopher_fisher/32/26132_2.png) [@Christopher\_Fisher](https://discourse.julialang.org/u/Christopher_Fisher)\
**Post date:** [November 12, 2018, 1:07pm UTC](https://discourse.julialang.org/t/type-stability-of-different-for-loops/17431/1 "2018-11-12T13:07:25Z")

</div>

Hi all-

I am experiencing a type instability problem with my code. I was able to narrow down the problem and find a solution, but it doesn’t make sense to me. It seems that iterating over objects rather than indices is the culprit. Below, I posted simplified examples in which `@code_warntype` reveals a problematic `Union`. I am wondering why this is happening and if it is an inherent limitation of type inference.

```julia
using Combinatorics,StatsFuns

#not type stable 
#Union{Nothing, Tuple{Tuple{Int64,Array{Bool,1}},Tuple{Int64,Array{Int64,1}}}}
function myFun1(n1,n2)
    M = multiset_permutations([fill(true,n1);fill(false,n2)],n1+n2)
    LLs = fill(0.0,length(M))
    for (i,m) in enumerate(M)
        m
    end
    return logsumexp(LLs)
end

#not type stable 
#Union{Nothing, Tuple{Tuple{Int64,Array{Bool,1}},Tuple{Int64,Array{Int64,1}}}}
function myFun2(n1,n2)
    M = multiset_permutations([fill(true,n1);fill(false,n2)],n1+n2)
    LLs = fill(0.0,length(M))
    for m in M
        m
    end
    return logsumexp(LLs)
end

#type stable 
function myFun3(n1,n2)
    M = collect(multiset_permutations([fill(true,n1);fill(false,n2)],n1+n2))
    LLs = fill(0.0,length(M))
    for i in 1:length(M)
        M[i]
    end
    return logsumexp(LLs)
end

```

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<div class="post-metadata">

**Author:** ![mauro3](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mauro3/32/292_2.png) [@mauro3](https://discourse.julialang.org/u/mauro3)\
**Post date:** [November 12, 2018, 1:26pm UTC](https://discourse.julialang.org/t/type-stability-of-different-for-loops/17431/2 "2018-11-12T13:26:04Z")

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Your loops don’t do anything, right? Also what are n1 and n2? Presumably Ints? `myFun3` does not work for me on Julia 1.0:

```julia
julia> myFun3(n1,n2)
ERROR: MethodError: no method matching getindex(::Combinatorics.MultiSetPermutations{Array{Bool,1}}, ::Int64)
Stacktrace:
 [1] myFun3(::Int64, ::Int64) at ./REPL[9]:5
 [2] top-level scope at none:0

```

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<div class="post-metadata">

**Author:** ![Christopher\_Fisher](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/christopher_fisher/32/26132_2.png) [@Christopher\_Fisher](https://discourse.julialang.org/u/Christopher_Fisher)\
**Post date:** [November 12, 2018, 1:42pm UTC](https://discourse.julialang.org/t/type-stability-of-different-for-loops/17431/3 "2018-11-12T13:42:13Z")

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My apologies. I forgot to add `collect()` to the multisets in myFun3. I corrected the initial post and it now works as intended. The type instability problem did not depend on the code in the for loop so I removed it to simplify the example.

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<div class="post-metadata">

**Author:** ![Christopher\_Fisher](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/christopher_fisher/32/26132_2.png) [@Christopher\_Fisher](https://discourse.julialang.org/u/Christopher_Fisher)\
**Post date:** [November 12, 2018, 1:44pm UTC](https://discourse.julialang.org/t/type-stability-of-different-for-loops/17431/4 "2018-11-12T13:44:28Z")

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Yes, n1 and n2 are Integers.