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# Syntactic quirks

We capture here some of the caveats and syntactic sugars here.

```
module Lang.syntaxQuirks where
open import Lang.dataStructures
open import Agda.Builtin.Equality
```

# Syntactic sugars

Mostly short-forms of various stuff used more often.

## Common parameters

The implicit parameter `{m : ℕ}`

common to all constructors can be abstracted out into the data definition:

```
data _≤′_ : ℕ → ℕ → Set where
≤′-refl : {m : ℕ} → m ≤′ m
≤′-step : {m : ℕ} → {n : ℕ} → m ≤′ n → m ≤′ succ n
```

is same as

```
data _≤′₁_ (m : ℕ) : ℕ → Set where
≤′₁-refl : m ≤′₁ m
≤′₁-step : {n : ℕ} → m ≤′₁ n → m ≤′₁ succ n
```

## Different ways of defining `data`

The previous technique also works for concrete parameters:

```
data _≤″_ : ℕ → ℕ → Set where
≤″ : ∀ {m n k} → m + n ≡ k → m ≤″ k
```

is same as

```
data _≤″₁_ (m : ℕ) : ℕ → Set where
≤+ : ∀ {n k} → m + n ≡ k → m ≤″₁ k
```

which is same as

```
data _≤″₂_ (m : ℕ) (k : ℕ) : Set where
≤+ : ∀ {n} → m + n ≡ k → m ≤″₂ k
```

## Implicit arguments

Arguments that can be inferred by the compiler can be left out with an `_`

:

```
length : (A : Set) (len : ℕ) → Vec A len → ℕ
length A zero [] = zero
length A len somevec = len
```

```
length' : (A : Set) (len : ℕ) → Vec A len → ℕ
length' A zero [] = zero
length' A len _ = len
```

```
length'' : (A : Set) (len : ℕ) → Vec A len → ℕ
length'' A zero [] = zero
length'' _ len _ = len
```

Though these are generally ill-advised as they may cause confusion when used unwisely.

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