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Fix issues in typesafe interpreter article.

This commit is contained in:
Danila Fedorin 2020-08-12 15:43:22 -07:00
parent 020417e971
commit 49469bdf12
1 changed files with 5 additions and 5 deletions
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content/blog/typesafe_interpreter_tuples.md
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@ -30,7 +30,7 @@ impossible to manually test every possible case in our typechecker,
but our approach of returning `Dec (a = b)` will work just fine. but our approach of returning `Dec (a = b)` will work just fine.
### Extending The Syntax ### Extending The Syntax
First, let's extend our existing language with expressions fpr First, let's extend our existing language with expressions for
tuples. For simplicity, let's use pairs `(a,b)` instead of general tuples. For simplicity, let's use pairs `(a,b)` instead of general
`n`-element tuples. This would make typechecking less cumbersome while still `n`-element tuples. This would make typechecking less cumbersome while still
having the interesting effect of making the number of types in our language having the interesting effect of making the number of types in our language
@ -46,7 +46,7 @@ Our `Expr` data type, which allows ill-typed expressions, ends up as follows:
I've highlighted the new lines. The additions consist of the `Pair` constructor, which I've highlighted the new lines. The additions consist of the `Pair` constructor, which
represents the tuple expression `(a, b)`, and the `Fst` and `Snd` constructors, represents the tuple expression `(a, b)`, and the `Fst` and `Snd` constructors,
which represent the `fst e` and `snd e` expressions, respectively. In which represent the `fst e` and `snd e` expressions, respectively. In
a similar vein, we extend our `SafeExpr` GADT: a similar manner, we extend our `SafeExpr` GADT:
{{< codelines "Idris" "typesafe-interpreter/TypesafeIntrV3.idr" 41 49 "hl_lines=7 8 9" >}} {{< codelines "Idris" "typesafe-interpreter/TypesafeIntrV3.idr" 41 49 "hl_lines=7 8 9" >}}
@ -93,7 +93,7 @@ Lines 14 through 16 are pretty simple; in this case, we can tell at a glance
that the two types are equal, and Idris can infer an equality proof in that the two types are equal, and Idris can infer an equality proof in
the form of `Refl`. We return this proof by writing it in a `Just`. the form of `Refl`. We return this proof by writing it in a `Just`.
Line 23 is the catch-all case for any combination of types we didn't handle. Line 23 is the catch-all case for any combination of types we didn't handle.
Any combination of types we don't handle is false, and thus, this case Any combination of types we don't handle is invalid, and thus, this case
returns `Nothing`. returns `Nothing`.
What about lines 17 through 22? This is the case for handling the equality What about lines 17 through 22? This is the case for handling the equality
@ -131,7 +131,7 @@ we want to substitute? After all, of the following are possible:
```Idris ```Idris
PairType rt1 lt2 = PairType lt1 lt2 -- First lt1 replaced PairType rt1 lt2 = PairType lt1 lt2 -- First lt1 replaced
PairType lt1 lt2 = PairType rt1 lt2 -- Second lt2 replaced PairType lt1 lt2 = PairType rt1 lt2 -- Second lt1 replaced
PairType rt1 lt2 = PairType rt1 lt2 -- Both replaced PairType rt1 lt2 = PairType rt1 lt2 -- Both replaced
``` ```
@ -147,7 +147,7 @@ t1 => PairType lt1 lt2 = PairType t1 lt2
t1 => PairType t1 lt2 = PairType t1 lt2 t1 => PairType t1 lt2 = PairType t1 lt2
``` ```
The highlighted function is the one we'll need to use to attain The second function (highlighted) is the one we'll need to use to attain
the desired result. Since `P` is an implicit argument to `replace`, the desired result. Since `P` is an implicit argument to `replace`,
we can explicitly provide it with `{P=...}`, leading to the following we can explicitly provide it with `{P=...}`, leading to the following
line: line: