Lesson 3 - unsure how "sigs" is created or where txInfoSignatories comes from or how it works

Question

Spoiler Alert!

The solution for lesson 3 includes the following:

{-# INLINABLE mkValidator #-}
-- This should validate if either beneficiary1 has signed the transaction and the current slot is before or at the deadline
-- or if beneficiary2 has signed the transaction and the deadline has passed.
mkValidator :: VestingDatum -> () -> ScriptContext -> Bool
mkValidator dat () ctx
    | (unPaymentPubKeyHash (beneficiary1 dat) `elem` sigs) && (to       (deadline dat) `contains` range) = True
    | (unPaymentPubKeyHash (beneficiary2 dat) `elem` sigs) && (from (1 + deadline dat) `contains` range) = True
    | otherwise = False
  where
    info :: TxInfo
    info = scriptContextTxInfo ctx
sigs :: [PubKeyHash]
sigs = txInfoSignatories info

range :: POSIXTimeRange
range = txInfoValidRange info

It appears that " txInfoSignatories " is a function (functor?) that analyzes " info " which itself is drawn from the script context, but I am at a loss as to where the " txInfoSignatories " code actually comes from. Is there a way to analyze which imported module it is attached to? Is there a way to analyze its code in order to understand what it does? Much of what I am seeing in Haskell seems so opaque, with new features simply appearing and magically doing something, but I am lost as to how I could have analyzed the code/modules/options and come up with this solution on my own.

For the record, here is what I created before I looked at the solution. It compiles without error, but does not behave in the playground as intended.

{-# INLINABLE mkValidator #-}
-- This should validate if either beneficiary1 has signed the transaction and the current slot is before or at the deadline
-- or if beneficiary2 has signed the transaction and the deadline has passed.
mkValidator :: VestingDatum -> () -> ScriptContext -> Bool
-- mkValidator _ _ _ = False -- FIX ME!
mkValidator dat () ctx = 
     ( traceIfFalse "beneficiary 1 - signature missing" signedByBeneficiary1 
    && traceIfFalse "deadline 1 has passed" deadline1Reached )
    || (traceIfFalse "beneficiary 2 - signature missing" signedByBeneficiary2 
    && traceIfFalse "deadline 2 not reached" deadline2Reached )
  where
    info :: TxInfo
    info = scriptContextTxInfo ctx
signedByBeneficiary1 :: Bool
signedByBeneficiary1 = txSignedBy info $ unPaymentPubKeyHash $ beneficiary1 dat

signedByBeneficiary2 :: Bool
signedByBeneficiary2 = txSignedBy info $ unPaymentPubKeyHash $ beneficiary2 dat

deadline1Reached :: Bool
deadline1Reached = contains (to $ deadline dat) $ txInfoValidRange info
deadline2Reached :: Bool
deadline2Reached = contains (from $ deadline dat) $ txInfoValidRange info

Any Haskell wisdom is much appreciated. I feel like this is more a Haskell knowledge deficit than anything else.

Answer 1

TxInfo is a Record type and one of the fields of the TxInfo-record is txInfoSignatories. The fields in the records work as "getters". You may refer e.g. to learnyouhaskell-records for an example.

The whole definition for TxInfo is here:

It is also completely fine to use the txSignedBy helper function as you have done to check whether the given hash is one of the signatories.