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+title = "Nockmas 2025: Day 8"
+date = "2026-01-01"
+description = "12 days of Nockmas: Compose, opcode 7"
+# aliases = []
+
+[extra]
+# author = ""
+ship = "~lagrev-nocfep"
+image = "https://s3.us-east-1.amazonaws.com/urbit.orgcontent/Blog/day+8/nockmas-day-8-compose-social.png"
+# imageCard = "https://s3.us-east-1.amazonaws.com/urbit.orgcontent/Blog/Blog_Building+on+Urbit_Social+16x9.png"
+# imageIndex = "https://s3.us-east-1.amazonaws.com/urbit.orgcontent/Blog/Blog_Building+on+Urbit_Tall+Banner.png"
+# imageDark =
+# imageCardDark =
+# imageIndexDark =
+tags =  ["nock", "nockmas", ""]
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+
+![nockmas day 8 compose image](https://s3.us-east-1.amazonaws.com/urbit.orgcontent/Blog/day+8/nockmas-day-8-compose-social.png)
+
+On this eighth day of Nockmas (and first of the New Year), we look at opcode 7, Compose. 
+
+# Opcode 7: Compose
+
+## Syntax
+
+```nock
+*[a 7 b c]          *[*[a b] c]
+```
+
+## Explanation
+
+Opcode 7 provides function composition. It evaluates `b` against the subject, then uses that result as the subject for evaluating `c`. This produces a classic “pipe” pattern.
+
+1. Evaluate `b` against subject to produce an intermediate noun.
+2. Evaluate `c` against that intermediate to produce the final product.
+
+When reading it, think of it as “then” or “pipe”.  The first formula changes the subject and the second operates on the new subject.
+
+Again, our subject:
+```nock
+:subject 42
+```
+
+**Output:**
+```
+Subject set to: 42
+```
+
+There are two common patterns for using opcode 7.  The first is simply to chain multiple computations together in sequence:
+
+```nock
+*[x [7 transform1 [7 transform2 transform3]]]
+```
+
+In the most simple case; increment, then increment again:
+
+```nock
+[7 [4 0 1] [4 0 1]]
+```
+
+**Output:**
+```
+44
+```
+
+Or continuing the chain: Increment, then increment again, then check if it's a cell:
+
+```nock
+[7 [4 0 1] [7 [4 0 1] 3 0 1]]
+```
+
+**Output:**
+```
+1
+```
+
+Make a cell of the subject repeated, then check if it's a cell:
+
+```nock
+[7 [[0 1] [0 1]] 3 0 1]
+```
+
+**Output:**
+```
+0
+```
+
+The second is to set up a context for a computation by first extending the subject with additional data, then performing the computation in that extended context:
+
+```nock
+*[input [7 [build-context] [main-computation]]]
+```
+
+(This is typically paired with [opcode 8](https://nock.is/content/specification/opcode-8.html) to build the context.)
+
+Revisiting our basic subject:
+```nock
+:subject 42
+```
+
+**Output:**
+```
+Subject set to: 42
+```
+
+We then call a Nock 7 to build a context and run a computation:
+```nock
+[7 [[0 1] [1 41]] [5 [0 2] [4 0 3]]]
+```
+
+**Output:**
+```
+0
+```
+
+Think through the formula above; What is it doing? 
+
+Having trouble? Shoot a note to [@urbit](https://x.com/urbit) or join us on Tlon Messenger in the [\[battery payload\] group](https://join.tlon.io/0v1.thdiu.q9kem.a0gtu.438pi.544q5).
+
+Join us tomorrow when we cover Nock 8, Extend.
+
+> *12 Days of Nockmas is an exploration of Nock, Urbit's instruction set architecture. This ISA is used by both Urbit and Nockchain, [has interpreters written in many languages](https://docs.urbit.org/nock/implementations), with production versions in both C and Rust. The content of this series is drawn from the [Nock language site](https://nock.is/). Visit the site for interactive code examples and more Nock related content.*