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# [BQN](https://mlochbaum.github.io/BQN/), 18 bytes<sup>[SBCS](https://github.com/mlochbaum/BQN/blob/master/commentary/sbcs.bqn)</sup>

```
{⊑(⊑𝕨⊸⍷)¨⊸/∾⟜𝕩¨↑𝕨}
```

[Run online!](https://mlochbaum.github.io/BQN/try.html#code=RuKGkHviipEo4oqR8J2VqOKKuOKNtynCqOKKuC/iiL7in5zwnZWpwqjihpHwnZWofQoKRsK0wqgg4p+oCiAgIkFCQ0RFRiLigL8iRUZHSEkiCiAgIkFBQUFBQSLigL8iQUFBQUFBQUEiCiAgIkFCQyLigL8iMTIzIgogICIi4oC/IkFCQ0RFIgogICJBQkNEIuKAvyJBQkNEIgogICIi4oC/IiIK4p+p)

A direct translation of Razetime's APL solution (my attempted improvement `⊣∾{⊢´/𝕨⊸«⊸≡¨↑𝕩}↓⊢` turns out to be not at all correct).

The BQN solution is much shorter mainly because it has [Prefixes](https://mlochbaum.github.io/BQN/doc/prefixes.html) (`↑`) built in. Being able to filter with `⊸/` (see [Before](https://mlochbaum.github.io/BQN/tutorial/combinator.html#before-and-after)) also helps a lot.

```
{⊑(⊑𝕨⊸⍷)¨⊸/∾⟜𝕩¨↑𝕨}  # Function with left argument 𝕨 and right argument 𝕩
               ↑𝕨   # All prefixes of 𝕨
           ∾⟜𝕩¨     # Append 𝕩 after each one
         ⊸/         # Filter by...
  (    )¨           #   On each string,
    𝕨⊸⍷             #     Where does 𝕨 appear as a substring?
   ⊑                #     But I only care if it's the first one
 ⊑                  # Then take the first
```

Put together, the pattern `⊑𝕨⊸⍷` tests if `𝕨` is a prefix of the argument.