# Allow units of measure in for loops #534

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opened this Issue Feb 9, 2017 · 1 comment

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commented Feb 9, 2017 edited

# Allow units of measure in for loops

I propose we extend the for loop typing to allow looping over an integer with units of measure.
Currently, the following won't compile:

``````let maximumDistance = 3<km>
let minimumDistance = 1<km>
for trial = minimumDistance to maximumDistance do
goRunning (trial * 1<km>)
``````

The existing way of approaching this problem is to cast away the units of measure, and multiply them back in within the loop, which is ugly and reduces the safety that uom adds.

## Pros and Cons

• An improvement to the usefulness of units of measure

• Maybe this would make loop optimization more tricky?

## Extra informtion

Estimated cost (XS, S, M, L, XL, XXL):
S to M ?

Related suggestions: (put links to reated suggestions here)
Allow units of measure in sequence ranges #535

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 As someone mentioned in #535, the `for` loop in this suggestion can currently be written as follows: ```let maximumDistance = 3 let minimumDistance = 1 for trial in minimumDistance .. 1 .. maximumDistance do goRunning trial``` Note that there's no need to cast `trial` inside the loop in this version, because it now has the unit of measure `km`. However, it would still be nice to be able to do `for trial in minimumDistance .. maximumDistance` or `for trial = minimumDistance to maximumDistance` without having to explicitly specify the step as a UoM. This seems like a case where the compiler should be able to Do The Right Thing automatically, inferring the correct unit of measure for `trial` and for the step.