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# Destination Calculus:
## A Linear 𝜆-Calculus for Purely Functional Memory Writes
### Topics in Compiler and Concurrency
#### Julius Fischer

###### 10. Juni 2026

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# What are Destinations?
- Destination Passing Style
    - Imperative
    - Function takes destination instead of returning structure
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# Example in C
```
int* vec_add(int v1[], int v2[], int size) {
    int* res =  malloc(size * sizeof(int));
    for (int i = 0; i < size; i++) {
        res[i] = v1[i] + v2[i];
    }
    return res;
}
```
- Add an 'out parameter'
```
void dps_vec_add(int res[], int v1[], int v2[], int size) {
    for (int i = 0; i < size; i++) {
        res[i] = v1[i] + v2[i];
    }
}
```
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# Motivation for DPS
- Typical functional map implementation
```
map f (x:xs) = (f x : map f xs1)
> (f x : map f xs) 
> (map f x1 : (f x2 : map f xs2)) 
> ...
```
- Not tail recursive 
    - Performance issue
    - Inefficient stack growth
```
map f (x:xs) dest = map f xs (push dest (f x))
> map f xs1 (push dest (f x1))
> map f xs2 (push dest (f x2))
```
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# DPS in functional languages
- Just add pointers to mutable data?
    - $Int \mapsto Int \mapsto Int \mapsto \textcolor{red}{T}$
    - $\textcolor{red}{T *}\mapsto Int \mapsto Int \mapsto Int \mapsto Void$

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# Destination Calculus
- It's 

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# Referenes