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Andreas M. Antonopoulos 2017-01-10 17:35:32 +07:00
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@ -344,7 +344,7 @@ When interpreting nSequence as a relative timelock, only the 16 least significan
The following diagram shows the binary layout of the nSequence value, as defined by BIP-68: The following diagram shows the binary layout of the nSequence value, as defined by BIP-68:
.BIP-68 definition of nSequence encoding .BIP-68 definition of nSequence encoding
image::nSequence_encoding.png["BIP-68 definition of nSequence encoding"] image::images/nSequence_encoding.png["BIP-68 definition of nSequence encoding"]
Relative timelocks based on consensus enforcement of the nSequence value are defined in BIP-68: Relative timelocks based on consensus enforcement of the nSequence value are defined in BIP-68:
@ -434,9 +434,9 @@ code to run in either case
When reading Bitcoin Script, remember that the condition being evaluated comes *before* the +IF+ opcode. When reading Bitcoin Script, remember that the condition being evaluated comes *before* the +IF+ opcode.
==== Flow Control with VERIFY opcodes ==== Conditional clauses with VERIFY opcodes
Another form of flow control in Bitcoin Script is any opcode that ends in +VERIFY+. The +VERIFY+ suffix means that if the condition evaluated is not TRUE, execution of the script terminates immediately and the transaction is deemed invalid. Another form of conditional in Bitcoin Script is any opcode that ends in +VERIFY+. The +VERIFY+ suffix means that if the condition evaluated is not TRUE, execution of the script terminates immediately and the transaction is deemed invalid.
Unlike an +IF+ clause which offers alternative execution paths, the +VERIFY+ suffix acts as a _guard clause_, continuing only if a precondition is met. Unlike an +IF+ clause which offers alternative execution paths, the +VERIFY+ suffix acts as a _guard clause_, continuing only if a precondition is met.
@ -475,7 +475,7 @@ Bob's unlocking script is identical:
The script with +IF+ does the same thing as using an opcode with a +VERIFY+ suffix, they both operate as guard clauses. However, the +VERIFY+ construction is more efficient, using one fewer opcode. The script with +IF+ does the same thing as using an opcode with a +VERIFY+ suffix, they both operate as guard clauses. However, the +VERIFY+ construction is more efficient, using one fewer opcode.
So, when do we use +VERIFY+ and when do we use +IF+? If all we are trying to do is to attach a pre-condition (guard clause), then +VERIFY+ is better. If however, we want to have more than one execution path, then we need an +IF...ELSE+ flow control clause. So, when do we use +VERIFY+ and when do we use +IF+? If all we are trying to do is to attach a pre-condition (guard clause), then +VERIFY+ is better. If however, we want to have more than one execution path (flow control), then we need an +IF...ELSE+ flow control clause.
==== Using Flow Control in Scripts ==== Using Flow Control in Scripts