# Frontend Emission Demos

These examples show the layer between source-level View annotations and the
portable View Contract checker.

Each `*.source.txt` file is pseudo-source: it is not parsed by `tricu`. It shows
the information a frontend has after parsing/elaboration.

Each matching `*.emitted.tri` file shows the lowered typed-program metadata that
a frontend can emit today. A successful check returns checked-exec; these demos
focus on structural Views, so they report `"ok"` as soon as metadata checking
succeeds. Guarded programs should run the returned checked-exec with
`runChecked`, as shown in `demos/viewContracts.tri` and by `tricu check`.

## Successful map use

Pseudo-source:

```text
map : Fn [Fn [Bool] String, List Bool] (List String)
f   : Fn [Bool] String
xs  : List Bool

partial = map f
out     = partial xs

require out : List String
```

Run the emitted artifact:

```bash
tricu eval demos/viewContracts/frontendEmission/map-success.emitted.tri -f decode
```

Expected output:

```text
"ok"
```

## Wrong list argument

Pseudo-source:

```text
map : Fn [Fn [Bool] String, List Bool] (List String)
f   : Fn [Bool] String
xs  : List String

partial = map f
out     = partial xs
```

Run:

```bash
tricu eval demos/viewContracts/frontendEmission/map-wrong-list.emitted.tri -f decode
```

Expected output:

```text
"symbol 162 expected List Bool but got List String"
```

## Wrong filter predicate

Pseudo-source:

```text
filter : Fn [Fn [Bool] Bool, List Bool] (List Bool)
pred   : Fn [Bool] String
xs     : List Bool

partial = filter pred
out     = partial xs
```

Run:

```bash
tricu eval demos/viewContracts/frontendEmission/filter-wrong-predicate.emitted.tri -f decode
```

Expected output:

```text
"symbol 181 expected Fn [Bool] Bool but got Fn [Bool] String"
```

## Lowering shape

A frontend does not need to expose `tricu` syntax internally. It only needs to
emit portable typed-program nodes:

```text
typedValue symbol view term
typedApply out callee arg term
typedRequire symbol view term
```

The source-level flow:

```text
out = map f xs
```

lowers to curried Tree Calculus application nodes:

```text
typedApply partial map f partialTerm
typedApply out partial xs outTerm
```

Function Views drive argument checking and result inference.