Switch starter.mojo kernels and solve functions from fn to def (#240)

Mojo 26.2 deprecates the `fn` keyword. Per the RFC "Remove fn from
Mojo" (accepted 23 Feb 2026), `def` is becoming Mojo's single function
declaration keyword:

- `def foo(...)` — non-raising (matches old `fn`)
- `def foo(...) raises` — raising form (replaces old `fn ... raises`
  and the old implicitly-raising `def`)

`fn` will be removed before 1.0. Starters emit a deprecation warning
on every compile today; refactor to the new form now so users don't
learn the dead keyword.

Kernels become `def` without `raises` (they don't raise). `solve`
becomes `def ... raises` since it calls DeviceContext, compile_function,
enqueue_function, and synchronize — all of which raise.

Verified against Mojo 26.2 on a Tesla T4: the transformed vector_add
starter compiles cleanly and produces a valid shared library.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: kunal-mansukhani

challenges/easy/19_reverse_array/starter/starter.mojo

@@ -4,13 +4,13 @@ from std.memory import UnsafePointer
 from std.math import ceildiv
 
 
-fn reverse_array_kernel(input: UnsafePointer[Float32, MutExternalOrigin], N: Int32):
+def reverse_array_kernel(input: UnsafePointer[Float32, MutExternalOrigin], N: Int32):
     pass
 
 
 # input is a device pointer (i.e. pointer to memory on the GPU)
 @export
-fn solve(input: UnsafePointer[Float32, MutExternalOrigin], N: Int32) raises:
+def solve(input: UnsafePointer[Float32, MutExternalOrigin], N: Int32) raises:
     var threadsPerBlock: Int32 = 256
     var ctx = DeviceContext()
 

challenges/easy/1_vector_add/starter/starter.mojo

@@ -4,7 +4,7 @@ from std.memory import UnsafePointer
 from std.math import ceildiv
 
 
-fn vector_add_kernel(
+def vector_add_kernel(
     A: UnsafePointer[Float32, MutExternalOrigin],
     B: UnsafePointer[Float32, MutExternalOrigin],
     C: UnsafePointer[Float32, MutExternalOrigin],
@@ -15,7 +15,7 @@ fn vector_add_kernel(
 
 # A, B, C are device pointers (i.e. pointers to memory on the GPU)
 @export
-fn solve(
+def solve(
     A: UnsafePointer[Float32, MutExternalOrigin],
     B: UnsafePointer[Float32, MutExternalOrigin],
     C: UnsafePointer[Float32, MutExternalOrigin],

challenges/easy/21_relu/starter/starter.mojo

@@ -4,7 +4,7 @@ from std.memory import UnsafePointer
 from std.math import ceildiv
 
 
-fn relu_kernel(
+def relu_kernel(
     input: UnsafePointer[Float32, MutExternalOrigin],
     output: UnsafePointer[Float32, MutExternalOrigin],
     N: Int32,
@@ -14,7 +14,7 @@ fn relu_kernel(
 
 # input, output are device pointers (i.e. pointers to memory on the GPU)
 @export
-fn solve(
+def solve(
     input: UnsafePointer[Float32, MutExternalOrigin],
     output: UnsafePointer[Float32, MutExternalOrigin],
     N: Int32,

challenges/easy/23_leaky_relu/starter/starter.mojo

@@ -4,7 +4,7 @@ from std.memory import UnsafePointer
 from std.math import ceildiv
 
 
-fn leaky_relu_kernel(
+def leaky_relu_kernel(
     input: UnsafePointer[Float32, MutExternalOrigin],
     output: UnsafePointer[Float32, MutExternalOrigin],
     N: Int32,
@@ -14,7 +14,7 @@ fn leaky_relu_kernel(
 
 # input, output are device pointers (i.e. pointers to memory on the GPU)
 @export
-fn solve(
+def solve(
     input: UnsafePointer[Float32, MutExternalOrigin],
     output: UnsafePointer[Float32, MutExternalOrigin],
     N: Int32,

challenges/easy/24_rainbow_table/starter/starter.mojo

@@ -4,7 +4,7 @@ from std.memory import UnsafePointer
 from std.math import ceildiv
 
 
-fn fnv1a_hash(input: Int32) -> UInt32:
+def fnv1a_hash(input: Int32) -> UInt32:
     alias FNV_PRIME: UInt32 = 16777619
     alias OFFSET_BASIS: UInt32 = 2166136261
 
@@ -17,7 +17,7 @@ fn fnv1a_hash(input: Int32) -> UInt32:
     return hash
 
 
-fn fnv1a_hash_kernel(
+def fnv1a_hash_kernel(
     input: UnsafePointer[Int32, MutExternalOrigin],
     output: UnsafePointer[UInt32, MutExternalOrigin],
     N: Int32,
@@ -28,7 +28,7 @@ fn fnv1a_hash_kernel(
 
 # input, output are device pointers (i.e. pointers to memory on the GPU)
 @export
-fn solve(
+def solve(
     input: UnsafePointer[Int32, MutExternalOrigin],
     output: UnsafePointer[UInt32, MutExternalOrigin],
     N: Int32,

challenges/easy/2_matrix_multiplication/starter/starter.mojo

@@ -4,7 +4,7 @@ from std.memory import UnsafePointer
 from std.math import ceildiv
 
 
-fn matrix_multiplication_kernel(
+def matrix_multiplication_kernel(
     A: UnsafePointer[Float32, MutExternalOrigin],
     B: UnsafePointer[Float32, MutExternalOrigin],
     C: UnsafePointer[Float32, MutExternalOrigin],
@@ -17,7 +17,7 @@ fn matrix_multiplication_kernel(
 
 # A, B, C are device pointers (i.e. pointers to memory on the GPU)
 @export
-fn solve(
+def solve(
     A: UnsafePointer[Float32, MutExternalOrigin],
     B: UnsafePointer[Float32, MutExternalOrigin],
     C: UnsafePointer[Float32, MutExternalOrigin],

challenges/easy/31_matrix_copy/starter/starter.mojo

@@ -4,7 +4,7 @@ from std.memory import UnsafePointer
 from std.math import ceildiv
 
 
-fn copy_matrix_kernel(
+def copy_matrix_kernel(
     A: UnsafePointer[Float32, MutExternalOrigin],
     B: UnsafePointer[Float32, MutExternalOrigin],
     N: Int32,
@@ -14,7 +14,7 @@ fn copy_matrix_kernel(
 
 # A, B are device pointers (i.e. pointers to memory on the GPU)
 @export
-fn solve(
+def solve(
     A: UnsafePointer[Float32, MutExternalOrigin],
     B: UnsafePointer[Float32, MutExternalOrigin],
     N: Int32,

challenges/easy/3_matrix_transpose/starter/starter.mojo

@@ -4,7 +4,7 @@ from std.memory import UnsafePointer
 from std.math import ceildiv
 
 
-fn matrix_transpose_kernel(
+def matrix_transpose_kernel(
     input: UnsafePointer[Float32, MutExternalOrigin],
     output: UnsafePointer[Float32, MutExternalOrigin],
     rows: Int32,
@@ -15,7 +15,7 @@ fn matrix_transpose_kernel(
 
 # input, output are device pointers (i.e. pointers to memory on the GPU)
 @export
-fn solve(
+def solve(
     input: UnsafePointer[Float32, MutExternalOrigin],
     output: UnsafePointer[Float32, MutExternalOrigin],
     rows: Int32,

challenges/easy/52_silu/starter/starter.mojo

@@ -4,7 +4,7 @@ from std.memory import UnsafePointer
 from std.math import ceildiv
 
 
-fn silu_kernel(
+def silu_kernel(
     input: UnsafePointer[Float32, MutExternalOrigin],
     output: UnsafePointer[Float32, MutExternalOrigin],
     N: Int32,
@@ -14,7 +14,7 @@ fn silu_kernel(
 
 # input, output are device pointers
 @export
-fn solve(
+def solve(
     input: UnsafePointer[Float32, MutExternalOrigin],
     output: UnsafePointer[Float32, MutExternalOrigin],
     N: Int32,

challenges/easy/54_swiglu/starter/starter.mojo

@@ -4,7 +4,7 @@ from std.memory import UnsafePointer
 from std.math import ceildiv
 
 
-fn swiglu_kernel(
+def swiglu_kernel(
     input: UnsafePointer[Float32, MutExternalOrigin],
     output: UnsafePointer[Float32, MutExternalOrigin],
     N: Int32,
@@ -14,7 +14,7 @@ fn swiglu_kernel(
 
 # input, output are device pointers
 @export
-fn solve(
+def solve(
     input: UnsafePointer[Float32, MutExternalOrigin],
     output: UnsafePointer[Float32, MutExternalOrigin],
     N: Int32,