# Surprising type inference failure for \`map\` on mid-size tuples

**URL:** <https://discourse.julialang.org/t/surprising-type-inference-failure-for-map-on-mid-size-tuples/88057>\
**Category:** General Usage\
**Created:** [September 30, 2022, 8:45pm UTC](https://discourse.julialang.org/t/surprising-type-inference-failure-for-map-on-mid-size-tuples/88057 "2022-09-30T20:45:39Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![nsajko](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nsajko/32/221187_2.png) [@nsajko](https://discourse.julialang.org/u/nsajko)\
**Post date:** [September 30, 2022, 8:45pm UTC](https://discourse.julialang.org/t/surprising-type-inference-failure-for-map-on-mid-size-tuples/88057/1 "2022-09-30T20:45:39Z")

</div>

This surprised me:

```julia
using Test # for Test.@inferred
g(::Val{n}) where {n} = ntuple(i -> one(Rational{BigInt}), Val{n}()) # constructs an example tuple of specified length
map(+, g(Val{30}()), g(Val{30}())); # warm up
@inferred map(+, g(Val{30}()), g(Val{30}())) # works fine
map(+, g(Val{40}()), g(Val{40}())); # warm up
@inferred map(+, g(Val{40}()), g(Val{40}())) # ERROR: return type NTuple{40, Rational{BigInt}} does not match inferred return type Tuple

```

I guess that using `ntuple` instead of `map` could be a workaround for this type inference issue.

But I’m curious, are there any similar gotchas that I should keep in mind?

---

<div class="post-metadata">

**Author:** ![nsajko](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nsajko/32/221187_2.png) [@nsajko](https://discourse.julialang.org/u/nsajko)\
**Post date:** [September 30, 2022, 10:48pm UTC](https://discourse.julialang.org/t/surprising-type-inference-failure-for-map-on-mid-size-tuples/88057/2 "2022-09-30T22:48:59Z")

</div>

Eventually I wrote these functions to fix my type inference issues (they’re an unary and a binary version):

```julia
# Like map, but with better type inference.
map_tuple(f::Fun, a::NTuple{len}) where {Fun <: Function, len} =
  ntuple(
    let a = a
      i -> f(a[i])
    end,
    Val{len}())

# Like map, but with better type inference.
map_tuple(f::Fun, a::T, b::T) where {Fun <: Function, len, T <: NTuple{len}} =
  ntuple(
    let a = a, b = b
      i -> f(a[i], b[i])
    end,
    Val{len}())

```

They work fine, but I wonder if a somewhat generalized version should really be in Julia `Base`? Pretty sure this is an issue that comes up often, and implementing a solution is out-of-reach for most newcomers.