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[Docs] [Builtins] Add Note [How to add a built-in function: simple cases] #4338

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[Docs] [Builtins] Add Note [How to add a built-in function: simple cases] #4338

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Add Note [How to add a built-in function: simple cases]
effectfully Jan 16, 2022
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Co-authored-by: Kenneth MacKenzie <kwxm@inf.ed.ac.uk>
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effectfully and Kenneth MacKenzie authored Jan 23, 2022
commit f6bc22af475edc2b4451d8fcd5ff00403f653c7e
6 changes: 3 additions & 3 deletions plutus-core/plutus-core/src/PlutusCore/Default/Builtins.hs
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Expand Up @@ -137,7 +137,7 @@ in relevant files (will have a proper overview doc on that, but for now you can
comment: https://github.com/input-output-hk/plutus/issues/4306#issuecomment-1003308938).

In order to add a new built-in function one needs to add a constructor to 'DefaultFun' and handle
it within the @ToBuiltinMeaning uni DefaultFun@ instance like that:
it within the @ToBuiltinMeaning uni DefaultFun@ instance like this:

toBuiltinMeaning <Name> =
makeBuiltinMeaning
Expand Down Expand Up @@ -168,7 +168,7 @@ builtin (out of the scope of this Note) and handling it within the @Flat Default
(see Note [Stable encoding of PLC]).

2. If the type of the denotation has any constrained type variables in it, all of them need to be
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Perhaps emphasise that this is the Haskell type of the denotation and that's where the type variables are coming from (I know the denotation's a Haskell object, but it's easy to get confused when there are two different type systems in play).

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I spent several minutes trying to come up with a way to make that clarification, but it just all looks worse. So I could say "If the Haskell type of the denotation has any constrained type variables in it", but that sounds like a denotation can have a non-Haskell type, which it can't. A builtin can have a Plutus type and a Haskell type, but a denotation is always a Haskell object. Plus, it's really unambiguous here which language is meant, because Plutus Core does not have any constraints. I agree it's easy to get confused in general, but I think in this particular case things are pretty unambiguous, given that the "denotation" term is introduced have a screen above and it's explicitly stated that a denotation is a Haskell implementation.

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But if you think there's a better way of formulating that part, do write it down, I'll include it in a subsequent PR, or feel free to adjust the docs yourself.

be instantiated. For example feeding @(+)@ directly to 'makeBuiltinMeaning' will give you an error
instantiated. For example feeding @(+)@ directly to 'makeBuiltinMeaning' will give you an error
message asking to instantiate constrained type variables, which you can do via an explicit type
annotation or type application or using any other way of specifying types.

Expand Down Expand Up @@ -289,7 +289,7 @@ the denotation is monomorphic or polymorphic w.r.t. failing.

But note that

'EvaluationResult' MUST BE EXPLICIYLY USED FOR ANY FAILING BUILTIN AND THROWING AN EXCEPTION
'EvaluationResult' MUST BE EXPLICITLY USED FOR ANY FAILING BUILTIN AND THROWING AN EXCEPTION
VIA 'error' OR 'throw' OR ELSE IS NOT ALLOWED AND CAN BE A HUGE VULNERABILITY. MAKE SURE THAT
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OR ELSE?

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errorWithoutStackTrace, throwTo, you name it.

NONE OF THE FUNCTIONS THAT YOU USE TO DEFINE A BUILTIN THROW EXCEPTIONS

Expand Down